How to Choose a Probiotic: A UK Buyer's Checklist

Written by Dr Zeeshan Afzal, MBBS — Medical Content Lead, Welzo. Last reviewed: July 2026.

Walk down any UK pharmacy aisle and you will find capsules promising 50 billion bacteria, shots promising 10 billion, and sachets promising "advanced multi-strain support". Almost none of them explain the one thing that actually predicts whether a product will help you. Learning how to choose a probiotic is less about chasing the biggest number on the box and more about matching a named bacterial strain to the specific outcome you want, at the dose that was actually tested in people.

This guide is a practical buyer's checklist written for UK shoppers. It explains what the label is really telling you, why British packaging often avoids the word "probiotic" altogether, how to judge CFU counts honestly, and how long to give a product before deciding it has failed. If you are starting further back, our overview of gut health in the UK and our guide to whether probiotics actually work are useful companions. You can also browse the full Welzo gut health range and our curated probiotics collection, which sit alongside targeted options such as Akkermansia muciniphila, modified citrus pectin powder, Welzo Ultra Purity Berberine and Welzo Ultra Purity TUDCA. For a shortlist of specific products, see our review of the best probiotics in the UK.

Table of contents

• The short answer
• What counts as a probiotic
• Why UK labels say "live cultures"
• Step 1: Start with the outcome
• Step 2: Check the strain, not the species
• Step 3: Judge the CFU dose honestly
• Step 4: Single strain or multi-strain
• Step 5: Survival and delivery claims
• Step 6: Choose a format you will stick with
• Step 7: Storage, shelf life and travel
• Step 8: Read the label like a pharmacist
• Step 9: Work out the real cost per day
• Step 10: Run a fair four-week trial
• Safety and who should ask first
• Choosing for specific groups
• When a probiotic is not the answer
• Frequently asked questions
• References

The short answer: how to choose a probiotic

To choose a probiotic well, work through six decisions in order:

1. Define the outcome. Bloating, stool regularity, cover during antibiotics and travel protection are different problems with different evidence bases.
2. Find a strain studied for that outcome. A strain is the full name plus its code, such as Lacticaseibacillus rhamnosus GG or Saccharomyces boulardii CNCM I-745.
3. Match the dose to the research. Check the CFU per serving against the amount used in trials of that strain, not against a rival brand's headline number.
4. Confirm potency at the end of shelf life. A guarantee "at time of manufacture" tells you almost nothing about what you will swallow in eight months.
5. Pick a format and storage you can live with. The best probiotic is one you take every day without thinking about it.
6. Give it at least four weeks. NICE advises people who choose to try probiotics for IBS to take one product for a minimum of four weeks at the manufacturer's dose while monitoring the effect.

Everything below expands on those six decisions, plus the label details that separate a serious product from an expensive one.

What actually counts as a probiotic

The definition scientists and regulators use

The internationally accepted definition comes from a Food and Agriculture Organization and World Health Organization consultation, reaffirmed in 2014 by an expert panel convened by the International Scientific Association for Probiotics and Prebiotics: probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.¹

Three words in that definition do a lot of work. Live means dead cells and fragments are something else entirely (they are usually called postbiotics). Adequate amounts means dose matters, and it is strain-specific rather than universal. Health benefit means the benefit has to have been demonstrated, not assumed. A jar of bacteria with no evidence attached to it is a jar of bacteria, not a probiotic.

This is why the question of prebiotics versus probiotics matters so much when you shop. Prebiotics are fibres that feed the microbes you already have; probiotics add microbes. Products that combine both are described as synbiotics, and a good one specifies both the strain and the fibre.

Why a UK label often avoids the word "probiotic"

Here is a quirk that catches out almost every British shopper. The Department of Health and Social Care treats the word "probiotic" as a health claim, because it implies that consuming those bacteria delivers a health benefit.² Under the Great Britain nutrition and health claims register, only authorised claims may be used, and the advertising regulator applies the same logic to marketing.³ There are currently no authorised health claims for probiotics; the one closely related authorised claim concerns live yoghurt cultures improving lactose digestion.⁴

The practical consequences for you as a buyer are worth spelling out:

• Compliant UK products often say "live cultures", "friendly bacteria" or simply name the strains, rather than saying "probiotic". This is a sign of regulatory care, not of a weaker product.
• Products that make bold, specific promises — curing bloating, treating IBS, fixing anxiety — are more likely to be breaching the rules than to be better formulated. Industry bodies have been lobbying for the terminology to change, but the current position stands.⁵
• Because supplements are regulated as foods rather than medicines, no UK authority verifies each batch's potency for you. That verification burden sits with the manufacturer, and with you.

Treat a vague label as a red flag and a specific, restrained one as a green flag. That single reframing will improve your shortlist more than any other tip in this article.

Step 1: Start with the outcome, not the bottle

Probiotics are not interchangeable, and "gut health" is not a target. Different strains do different things, and the effect sizes vary from meaningful to negligible depending on what you are asking of them.

Where the evidence is strongest and weakest

The American Gastroenterological Association reviewed probiotics across eight gastrointestinal conditions using GRADE methodology and concluded that, for most digestive conditions, the evidence does not support routine use. It supported specific formulations in three settings: preventing Clostridioides difficile infection in adults and children taking antibiotics, preventing necrotising enterocolitis in preterm low-birthweight infants, and managing pouchitis.⁶ For IBS, it recommended probiotics only within a clinical trial, and it suggested against their use in children with acute infectious gastroenteritis.⁷

That is a deliberately conservative reading, and other bodies are more permissive. NICE explicitly allows for people with IBS who choose to try probiotics, with the four-week rule attached.⁸ The British Dietetic Association's position is more blunt: if you are healthy, a probiotic supplement is unlikely to benefit you, and if you have a gut condition you should speak to a dietitian who can point you towards the product with the best evidence for your symptoms.⁹ Guts UK, the digestive charity, notes that research evidence for short and long-term probiotic use remains limited.¹⁰

Cover during and after antibiotics

This is the use case with the most consistent support, and specific yeasts and bacterial combinations have been assessed for it. If this is your reason for buying, read our detailed guide to taking probiotics after antibiotics and our page on Saccharomyces boulardii in the UK, since a yeast behaves differently from a bacterium and is not killed by the antibiotic itself. There is also a dedicated overview of probiotics and C. difficile.

IBS, bloating and trapped wind

Results are strain-dependent and individual. Start with our guides to supplements for IBS, supplements for bloating and IBS subtypes, because a product that helps IBS with constipation may worsen IBS with diarrhoea. If bloating is your main complaint, rule out the mechanical causes first using our pages on trapped wind relief, foods that cause bloating and SIBO.

Travel and stomach upsets abroad

A different job again, usually requiring you to start before you fly. See probiotics for traveller's diarrhoea and travel probiotics, and note that shelf-stable formulations are far more practical in a suitcase.

General wellbeing and diversity

If you have no specific symptom, food usually beats capsules. Increasing plant variety, fibre and fermented foods has better support for improving microbiome diversity than any single supplement. Our guides to 30 plants a week, prebiotic foods, fermented foods in the UK and how to increase fibre are the better starting point.

Step 2: Check the strain, not just the species

This is the single most useful skill in probiotic shopping, and it takes about thirty seconds once you know what you are looking at.

Genus, species, strain

Take Lacticaseibacillus rhamnosus GG. Lacticaseibacillus is the genus. rhamnosus is the species. GG is the strain designation, and it is the part that links the product to published research. Two capsules both listing "L. rhamnosus" with no code may contain organisms that behave completely differently in your gut.

ISAPP, the field's main scientific body, advises choosing products backed by evidence for the benefit you want and checking the label for storage conditions and expiry.¹¹ The same organisation has highlighted survey work finding that around 45% of retail supplement products did not provide strain designations at all, a similar proportion gave no potency measure through the end of shelf life, and only about 35% could be linked to evidence of a health benefit on the basis of strain and dose.¹² In other words, roughly two thirds of what you can buy cannot be checked against research even in principle.

If you want to go deeper on individual organisms, we have dedicated pages on Lactobacillus rhamnosus GG, Bifidobacterium longum, Lactobacillus plantarum, Bacillus coagulans and spore-based probiotics.

The 2020 renaming that still confuses labels

In 2020, a large genomic analysis reclassified the sprawling genus Lactobacillus into 25 genera, introducing names such as Lacticaseibacillus, Lactiplantibacillus, Limosilactobacillus and Levilactobacillus.¹³ Species names and strain codes did not change.

So when you see one brand list Lactobacillus plantarum 299v and another list Lactiplantibacillus plantarum 299v, these are the same organism under old and new naming. Neither is wrong. If a label uses the current genus names, it suggests someone technical has reviewed the artwork recently. Our comparison of spore-forming versus Lactobacillus probiotics covers why these genus differences have practical consequences for survival and dosing.

Step 3: Judge the CFU dose honestly

What CFU actually measures

CFU stands for colony-forming units: the number of viable microbes capable of multiplying under laboratory conditions. It is a count of living cells at the moment of testing, and it is the closest thing the category has to a potency standard. Our full explainer on CFU versus AFU covers the newer flow-cytometry alternative, active fluorescent units, which counts cells differently and therefore cannot be compared like for like with CFU.

At manufacture or at the end of shelf life?

This distinction matters more than the headline number. Live bacteria die off gradually in storage. A product guaranteeing 50 billion CFU "at time of manufacture" may deliver a small fraction of that by its best-before date, whereas one guaranteeing 10 billion CFU "to end of shelf life" is committing to what you will actually swallow.

Independent testing supports the caution. A 2026 laboratory analysis of 28 commercially available probiotic products across capsules, powders, suspensions, drops and chocolate bars found that just under half showed CFU counts below their labelled values.¹⁴ That is a small sample and should not be read as a verdict on any particular brand, but it does explain why the potency guarantee wording is worth reading before you buy.

Is a higher CFU count better?

Not automatically. The right dose is the dose used in trials of that strain for that outcome, and for many well-studied strains that figure sits in the range of one to ten billion CFU per day rather than in the tens of billions. A 100 billion CFU product of an unnamed strain is not superior to a 10 billion CFU product of a strain with published trials behind it.

Very high doses are also more likely to produce short-term wind and bloating while you adjust. See our guides on taking too many probiotics, probiotic side effects and high-strength probiotics if you are considering a large dose, and how long probiotics take to work for realistic expectations.

Step 4: Single strain or multi-strain?

Multi-strain blends are marketed as more comprehensive, and that intuition is reasonable but not well proven. The practical trade-off is this: a single-strain product is easy to match to evidence, while a fifteen-strain blend usually cannot be matched to any trial because that exact combination at those exact doses has rarely been studied as a unit.

There is also a formulation problem. In a multi-strain product, different organisms decline at different rates in storage, so the ratio of strains at the end of shelf life may be nothing like the ratio at manufacture. That is one reason a blend's total CFU headline can be misleading.

A reasonable rule: if you are targeting a defined problem, favour the product whose strains you can look up. If you are supplementing a varied diet with no specific complaint, a blend is a perfectly sensible choice. Our comparison of single versus multi-strain probiotics goes into the evidence in more detail, and it pairs well with our guide to Akkermansia versus conventional probiotics if you are considering next-generation strains, or digestive enzymes versus probiotics if your symptoms centre on meals.

Step 5: Treat survival and delivery claims with measured scepticism

Stomach acid does kill a proportion of ingested bacteria, so delivery is a genuine engineering problem. Manufacturers address it with acid-resistant capsules, enteric coatings, microencapsulation, spore-forming organisms that are naturally acid-tolerant, and liquid formats designed to pass through the stomach quickly.

The caveat is that "survives stomach acid" on a box is a marketing claim, not a measurement. What would substantiate it is a published study on that specific product showing recovery of viable organisms further down the gut. Ask whether such a study exists rather than accepting the claim at face value. Our page on whether probiotics survive stomach acid covers the mechanisms, and when to take probiotics explains why timing around meals can matter more than coatings for some products.

Step 6: Choose a format you will actually stick with

Adherence beats sophistication. A £15 capsule you take every morning outperforms a £45 shot that sits at the back of the fridge.

A few format-specific points worth knowing:

• Capsules dominate the research literature and make dosing unambiguous. Check whether the capsule shell is gelatin or HPMC if you are vegan or vegetarian; see vegan probiotics.
• Powders and sachets suit children, anyone who dislikes swallowing pills, and dose-splitting when you are introducing a product slowly. Never stir them into anything hot. See probiotic powders and children's probiotics.
• Shots and drinks are convenient and some UK brands have their own trial data, but they are the most expensive format per day and the bulkiest to store. Our probiotic drinks guide and Symprove review compare the main options.
• Dairy-free formulations matter if you react to milk proteins, since some cultures are grown on dairy substrates. See dairy-free probiotics.
• Fermented foods such as kefir, live yoghurt, kimchi and sauerkraut are worth eating, but their live counts are rarely declared or guaranteed. Our comparison of fermented food versus probiotics and our guide to kefir benefits set out where each fits.

Step 7: Storage, shelf life and travel

The BDA's advice here is simple and often ignored: always store probiotic foods or supplements as the packaging instructs, because storage conditions affect how many live organisms remain when you consume the product.⁹

Modern freeze-drying and blister packaging mean many products no longer need refrigeration, and a shelf-stable product is not inherently inferior. What matters is that the product's own instructions are followed. Heat and humidity are the two biggest enemies, which is why a bathroom cabinet is one of the worst places to keep them. Our guides to whether probiotics need refrigeration and shelf-stable probiotics cover this in full.

Two practical checks before you buy:

• Is the best-before date far enough away that you will finish the pack well before it? Buying a large tub on offer is false economy if potency is declining throughout.
• If you travel or commute, does the format survive a warm bag? Blister-packed, ambient-stable capsules are the pragmatic choice.

Step 8: Read the label like a pharmacist

Use this comparison as a quick sorting tool in the aisle or on a product page.

If you are comparing named UK brands directly, our reviews of Optibac versus Bio-Kult, Alflorex versus Symprove, Symprove versus Optibac and Seed in the UK apply this same checklist product by product.

Step 9: Work out the real cost per day

Pack prices are designed to be hard to compare. Convert everything to cost per day over a full four-week trial before deciding.

Then apply a simple test: over four weeks, is this the best use of that money for your gut? For some people the answer is a probiotic. For others it is more fibre, a structured low FODMAP trial with a dietitian, or investigating an underlying cause via a gut microbiome test, coeliac test or SIBO breath test.

Step 10: Run a fair four-week trial

Most people abandon probiotics after a week, or take three products at once and learn nothing from any of them. NICE's advice for IBS is a good general template: take one product for at least four weeks at the manufacturer's recommended dose while monitoring the effect.⁸

To make the trial worth running:

• Change one thing at a time. Starting a probiotic, a fibre supplement and an elimination diet in the same week makes the result uninterpretable.
• Record a baseline first. Note symptom frequency and stool form for a week before you start, using the Bristol stool chart.
• Expect a settling-in period. Mild wind or looser stools in the first week are common and usually pass. Persistent or severe symptoms mean stop.
• Set a decision point. At four weeks, either continue because something measurably improved, or stop and try a different strain. Our guide to taking probiotics every day covers what happens after a successful trial.

Safety: who should check with a clinician first

For most healthy adults, probiotics are well tolerated, and the usual side effects are transient wind, bloating or a change in stool frequency during the first days. That said, they are not risk-free for everyone, and there are groups who should get individual advice before starting.

Speak to your GP, pharmacist or specialist before taking a probiotic if you:

• Are significantly immunocompromised, including during chemotherapy, after transplant, or with an immunodeficiency
• Are critically unwell, or have a central venous catheter or other indwelling line
• Have short bowel syndrome, a damaged gut barrier, or recent major abdominal surgery
• Have a heart valve abnormality or a history of endocarditis
• Are caring for a premature or very low-birthweight infant, where products should only be used under neonatal specialist supervision
• Take medicines that could interact, including immunosuppressants and antifungals

Our page on probiotic safety covers this in more depth, and probiotics in pregnancy addresses that specific situation. Probiotics are also not a substitute for prescribed treatment, and you should not stop medication such as a proton pump inhibitor without medical advice — see long-term effects of omeprazole.

Choosing for specific groups

The checklist above applies universally, but a few groups have extra considerations:

• Women. Some strains are studied for vaginal and urinary health as well as digestion, and hormonal transitions change gut function. See probiotics for women and gut health in menopause.
• Men. Priorities often centre on regularity, training load and metabolic health — see probiotics for men and gut health for athletes.
• Children and infants. Dose and format differ substantially from adult products; never split adult capsules. See children's probiotics and baby probiotics.
• Older adults. Medication burden and constipation are common complicating factors. See gut health in older adults and gut health and ageing.
• Shift workers and frequent travellers. Irregular timing undermines consistency; see shift work and digestion.

When a probiotic is not the answer

No supplement should delay assessment of symptoms that need investigating. Contact your GP promptly, rather than buying a probiotic, if you have:

• Blood in your stool, or black tarry stools
• Unintentional weight loss
• A persistent change in bowel habit lasting more than six weeks, particularly if you are over 50
• Difficulty swallowing, persistent vomiting, or a lump in your abdomen
• Unexplained iron-deficiency anaemia
• Severe or worsening abdominal pain, or symptoms that wake you at night
• A family history of bowel cancer, coeliac disease or inflammatory bowel disease alongside new symptoms

Our guides on when to see a GP about stomach symptoms and bowel cancer screening in the UK explain what happens next. If you have a diagnosed condition, dietary strategy usually matters more than supplementation — see our pages on Crohn's disease diet, ulcerative colitis diet and diverticulitis diet.

How probiotics fit into a wider gut plan

A probiotic is one tool, and rarely the first one. The interventions with the broadest support are dietary: more plant variety, more fibre, more fermented foods, less ultra-processed food, and better sleep and stress management. Our pages on the best foods for gut health, ultra-processed food and the gut, gut health and sleep and the gut-brain connection cover the foundations.

Where supplements do have a role, it is usually targeted rather than general. Depending on your situation that might mean a prebiotic, butyrate to support short-chain fatty acid production, zinc carnosine or collagen for gut barrier function, magnesium for constipation, or peppermint oil capsules for IBS pain. You can explore all of these in the Welzo gut health collection.

Frequently asked questions

How do I choose a probiotic that will actually work for me?

Choose by outcome first. Identify the specific symptom you want to change, find a strain with published trials for that symptom, check that the product delivers a comparable CFU dose per serving with potency guaranteed to the end of shelf life, then take it for at least four weeks while tracking your symptoms. If nothing changes, that strain is not the right one for you, and a different strain is a more sensible next step than a higher dose of the same one.

What CFU count should I look for in a probiotic?

There is no universal number. The right dose is whatever was used in research on that specific strain for your specific goal, which for many well-studied strains falls between one and ten billion CFU per day. A higher headline count from an unnamed strain is not better than a lower count from a strain you can look up. Always check whether the CFU figure is per serving or per pack, and whether it is guaranteed at manufacture or at expiry.

Why do UK probiotic labels say "live cultures" instead of "probiotic"?

Because the Department of Health and Social Care treats "probiotic" as a health claim, and there are no authorised probiotic health claims on the Great Britain register. Compliant UK brands therefore describe their products as live cultures or friendly bacteria, or simply list the strains. It is a labelling technicality rather than a sign of a weaker product, and a brand that respects it is often the more carefully run one.

Is a multi-strain probiotic better than a single-strain one?

Not necessarily. Multi-strain blends are harder to match to evidence because the specific combination and doses are rarely the ones tested in trials, and different strains decline at different rates during storage. Single-strain products are easier to verify against research. Choose single-strain when you have a defined goal, and consider a blend if you have no specific complaint.

Do probiotics need to be refrigerated?

Some do and many modern ones do not. Freeze-drying and moisture-barrier packaging have made shelf-stable formulations reliable. What matters is following the storage instructions on the specific pack, since storage conditions affect how many organisms remain alive. Keep any probiotic away from heat and humidity, which rules out the bathroom cabinet.

How long does it take for a probiotic to work?

Allow four weeks before judging. NICE advises people with IBS who choose to try probiotics to take one product for at least four weeks at the manufacturer's recommended dose while monitoring the effect. Some people notice changes in bowel habit within a week or two, but a fair trial needs the full period, with a single change at a time and a symptom record.

Can I take a probiotic while I am on antibiotics?

Many people do, and this is the use case with the most consistent evidence. Specific yeast and bacterial preparations have been assessed for reducing antibiotic-associated diarrhoea. Spacing the probiotic a few hours from the antibiotic dose is a common practical approach. If you are unwell, immunocompromised, or in hospital, ask the prescribing clinician first rather than self-treating.

Are probiotic side effects normal when starting?

Mild wind, bloating or a change in stool frequency during the first few days is common and usually settles. Starting at a lower dose and building up can reduce it. Stop and seek advice if symptoms are severe, if you develop a fever, or if symptoms persist beyond a couple of weeks. Anyone who is immunocompromised or seriously unwell should get medical advice before starting at all.

Are fermented foods as good as a probiotic supplement?

They are different tools. Fermented foods such as kefir, live yoghurt, kimchi and sauerkraut are worth eating and contribute to a varied diet, but they rarely declare which organisms they contain or how many survive to the point of eating, so they cannot be matched to specific research. Supplements offer defined strains and doses. Most people benefit more from adding fermented foods and plant variety than from any capsule.

Do healthy people need to take a probiotic at all?

Probably not routinely. The British Dietetic Association's position is that if you are healthy, a probiotic supplement is unlikely to be beneficial, and the American Gastroenterological Association found insufficient evidence to support probiotics for most digestive conditions. Probiotics make most sense when there is a defined goal, such as cover during antibiotics, travel, or a trial for specific IBS symptoms, rather than as a permanent daily habit without a reason.

References

1. Hill C, Guarner F, Reid G, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology, 2014. https://www.nature.com/articles/nrgastro.2014.66
2. UK Parliament. Written question on food labelling — Department of Health and Social Care response on the term "probiotic". https://questions-statements.parliament.uk/written-questions/detail/2026-03-26/HL16090/
3. Committee of Advertising Practice / Advertising Standards Authority. Food: Probiotic claims. https://www.asa.org.uk/advice-online/food-probiotic-claims.html
4. Food Safety Authority of Ireland. Probiotic health claims. https://www.fsai.ie/business-advice/nutrition/probiotic-health-claims
5. Proprietary Association of Great Britain (PAGB). Probiotics policy position. https://www.pagb.co.uk/food-supplements/probiotics/
6. American Gastroenterological Association. AGA does not recommend the use of probiotics for most digestive conditions. https://gastro.org/press-releases/aga-does-not-recommend-the-use-of-probiotics-for-most-digestive-conditions/
7. American Gastroenterological Association. Clinical practice guidelines on the role of probiotics in the management of gastrointestinal disorders. https://gastro.org/clinical-guidance/role-of-probiotics-in-the-management-of-gastrointestinal-disorders/
8. National Institute for Health and Care Excellence. Irritable bowel syndrome in adults: diagnosis and management (CG61), Recommendations. https://www.nice.org.uk/guidance/cg61/chapter/Recommendations
9. British Dietetic Association. Probiotics and gut health food fact sheet. https://www.bda.uk.com/resource/probiotics.html
10. Guts UK. Prebiotics and probiotics. https://gutscharity.org.uk/advice-and-information/health-and-lifestyle/prebiotics-probiotics/
11. International Scientific Association for Probiotics and Prebiotics. How to choose a probiotic. https://isappscience.org/resource/how-to-choose-a-probiotic/
12. International Scientific Association for Probiotics and Prebiotics. Decoding a probiotic product label. https://isappscience.org/decoding-a-probiotic-product-label/
13. Zheng J, Wittouck S, Salvetti E, et al. A taxonomic note on the genus Lactobacillus: description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae. International Journal of Systematic and Evolutionary Microbiology, 2020. https://www.microbiologyresearch.org/content/journal/ijsem/10.1099/ijsem.0.004107
14. Label accuracy of commercial probiotics — CFU enumeration and microbial composition. Annals of Microbiology, 2026. https://link.springer.com/article/10.1186/s13213-026-01854-w

About the author

Dr Zeeshan Afzal, MBBS is a qualified medical doctor and Medical Content Lead at Welzo, where he writes and reviews clinical content on digestive health, supplementation and preventive medicine for UK readers. He has clinical experience across general medicine and primary care, and reviews Welzo's digestive health library against UK guidance from NICE, the NHS and UK professional bodies.

Medical disclaimer

This article is for general information and does not replace individual medical advice. Food supplements are not medicines and should not be used to treat, prevent or cure disease. Always speak to your GP, pharmacist or a registered dietitian before starting a supplement, particularly if you are pregnant or breastfeeding, immunocompromised, taking prescription medicines, or living with a diagnosed digestive condition. If you have red flag symptoms such as rectal bleeding, unexplained weight loss or a persistent change in bowel habit, contact your GP promptly. In an emergency, call 999 or NHS 111.