Written and medically reviewed by Dr Zeeshan Afzal (MBBS) — Medical Doctor & Medical Content Lead, Welzo
Evidence-based · Honest about what's proven and what isn't · UK-focused
Last updated: July 2026 · Reading time: ~10 minutes
Part of our Digestive Health hub — buy gut health supplements
Popular probiotics: Akkermansia · Citrus Pectin · Berberine · TUDCAIn the best trial of the best-evidenced Bifidobacterium longum strain, a dose of 1×108 CFU improved IBS symptoms. A dose one hundred times larger did not beat placebo.
That single finding should change how you shop this category. "High strength" is the easiest thing for a brand to put on a box and one of the least informative numbers on it — and the number itself often isn't even measuring the same thing from one product to the next.
This guide explains what CFU counts actually mean, why 50 billion isn't the milestone it's marketed as, and ranks four probiotics by their declared potency. Where a listing doesn't state a figure, we say so. See also CFU vs AFU and gut health supplements.
Table of contents
Whorwell and colleagues, publishing in the American Journal of Gastroenterology in 2006, randomised 362 women in primary care with IBS to B. longum 35624 at one of three doses, or placebo, for four weeks.
At 1×108 CFU, the strain significantly improved abdominal pain, bloating, bowel dysfunction, flatulence, straining and incomplete evacuation. At 1×106 and 1×1010 CFU it did not. The highest dose — a hundred times the effective one — performed no better than placebo.
Cochrane's analysis of probiotics for preventing antibiotic-associated diarrhoea points the same way: it identified an effective range of roughly 5 to 40 billion CFU daily, not "as much as possible".
Even setting the evidence aside, comparing two potency figures is rarely comparing like with like.
| The variable | Why it breaks the comparison |
|---|---|
| CFU vs AFU | Different counting methods. AFU counts cells by fluorescence; CFU counts colonies that grow on a plate. Not interchangeable. |
| Spores vs freeze-dried | Spores survive the stomach almost intact, so 5 billion spores may deliver more live organisms than 50 billion freeze-dried cells. |
| At manufacture vs at expiry | A factory figure is not a promise about the capsule you swallow next year. |
| Per gram vs per capsule | Powders quote per gram; capsules quote per capsule. A serving may be a fraction of a gram. |
| Total vs per strain | A 50 billion, 10-strain blend may contain only 5 billion of the strain you actually want. |
That last row matters most. A high total across many strains can mean a low dose of each — the argument for single-strain products covered in single vs multi-strain probiotics. On spores specifically, see spore-based probiotics.
High-strength products are not pointless. There are situations where more is reasonable:
What is not supported is the idea that a healthy adult taking a probiotic for general maintenance needs 100 billion CFU rather than 10 billion.
No. It is a marketing round number, not a clinical one. No guideline identifies 50 billion CFU as a meaningful cut-off, and the trial evidence above suggests the useful range sits well below it for most indications.
It is also worth knowing that genuinely high-potency products are rarer than the marketing suggests. Across the range reviewed here, very few declare a figure above 50 billion at all — and several of the best-evidenced probiotics on the market sit at 10 to 25 billion quite deliberately, because that is where their trials were run.
If you want a specific researched strain at its trial dose, see Lactobacillus rhamnosus GG or Bifidobacterium longum. If you want the highest declared numbers available, read on.
Where a product does not declare a potency figure, we say so rather than estimating one.
Best Overall: Welzo Ultra Purity Akkermansia — £21.99
Highest Declared Potency: Custom Probiotics B. Longum Powder, 50g — £103.25
Best Multi-Strain: Custom Probiotics Adult Formula CP-1, 90 caps — £77.41
Best Single Strain: Pure Encapsulations ProFloraGG 25B, 30 caps — £20.47
Price: £21.99 (from £39.99) · Reviews: 92 · Stock: In stock · Declared potency: 60 billion AFU (note: AFU, not CFU) · Where to buy: Available from Welzo
Why it's the best: Three reasons, and the first is the one that decides it. It is the only high-potency product here that you can actually buy today — the two products with higher or comparable declared figures are both out of stock, and a probiotic you cannot purchase has no strength at all.
Second, the potency figure is directly usable. The 100 billion figure on the powder below is quoted per gram, not per serving — a typical scoop is a fraction of a gram, so the number on the tub is not what you swallow. This product declares 60 billion AFU per serving, which is the figure that actually reaches you. On a like-for-like per-dose basis the gap between them is far narrower than the headline numbers suggest.
Third, the price. At £21.99, reduced from £39.99, it costs a little over a fifth of the powder below and under a third of the multi-strain formula. For a category where the premium is usually charged for the number on the front, that is the strongest value here by a wide margin.
The formulation pairs Akkermansia at 60 billion AFU with Lactobacillus acidophilus and inulin, plus berberine, chromium and astaxanthin.
Akkermansia is also mechanistically distinct: it works on the intestinal mucus layer rather than the lumen, so it occupies a different niche from the lactobacilli in most high-strength blends — see Akkermansia vs probiotics.
| Pros | Cons |
|---|---|
| Highest-potency option that is in stock By far the best value (£21.99, from £39.99) Two organisms in different gut niches Includes inulin as a prebiotic Declares its potency figure clearly |
AFU is not directly comparable to CFU Multi-ingredient — not for purists Akkermansia evidence is younger than Lactobacillus Berberine has its own interaction profile |
Price: £103.25 · Reviews: 80 · Stock: Currently sold out — check listing · Declared potency: 100 billion CFU per gram · Where to buy: Available from Welzo
Why it ranks here: It declares the highest raw potency of anything reviewed, and — unusually for a powder — it actually publishes the figure. It would top this list on the number alone; it sits second because it is currently unavailable and because that number is quoted per gram rather than per serving. It also names its strain, BL-05, and is dairy-free, sugar-free, gluten-free and non-GMO with no capsule shell.
Read the unit carefully: that is 100 billion CFU per gram, not per serving. A typical serving is a fraction of a gram, so your actual daily dose depends entirely on how much you scoop. That is a feature rather than a flaw — the powder format lets you titrate from a very small dose upwards, which is genuinely useful if you react to probiotics. See probiotic powders.
It is expensive and currently unavailable. At £103.25 for 50g — about £2.07 a gram — it costs roughly four times per gram what a comparable single-species powder does. BL-05 is a properly characterised strain but does not carry the published record of the best-studied B. longum strains.
| Pros | Cons |
|---|---|
| Highest declared potency here (100 billion CFU/g) Actually publishes its figure Names its strain (BL-05) Dairy-free, sugar-free, gluten-free, non-GMO Dose can be titrated precisely |
Currently sold out Per gram, not per serving — real dose depends on your scoop ~£2.07 per gram BL-05 has a limited published record |
Price: £77.41 · Reviews: 152 · Stock: In stock · Declared potency: Not stated on the listing · Where to buy: Available from Welzo
Why we like it: A multi-strain acidophilus-and-bifidus formula in a 90-capsule pack — a three-month supply, which is longer than almost anything else here. It has 152 reviews, the highest count in this guide, and Custom Probiotics is a specialist brand whose range is built around high-potency, allergen-conscious formulations.
Combining Lactobacillus and Bifidobacterium species covers both the small and large intestine, which is the sensible rationale for a multi-strain product.
| Pros | Cons |
|---|---|
| 90 capsules — three-month supply Most reviewed product here (152) Covers both Lactobacillus and Bifidobacterium Specialist high-potency brand In stock |
No CFU figure declared on the listing Expensive at £77.41 Individual strain amounts not disclosed Cannot be compared on potency |
Price: £20.47 · Reviews: 138 · Stock: Currently sold out — check listing · Declared potency: 25 billion CFU per capsule · Where to buy: Available from Welzo
Why it earns a place despite the lowest number here: because 25 billion CFU of a strain you can actually look up beats 100 billion of one you can't. This is Lactobacillus rhamnosus GG — the most heavily researched probiotic strain in existence, with several hundred human trials and a place in clinical guidance for preventing antibiotic-associated diarrhoea. See our LGG guide.
The formulation is three ingredients — the strain, microcrystalline cellulose and an HPMC capsule — and it is shelf stable, gluten-free certified and dairy-free. At £20.47 it is also the cheapest option in this guide.
If you are buying on CFU alone, this is not your product. If you are buying on evidence per pound, it is comfortably the best here. It does go out of stock frequently.
| Pros | Cons |
|---|---|
| 25 billion CFU of the most researched strain available Declares potency clearly, per capsule Only three ingredients Cheapest option here (£20.47) Shelf stable, gluten-free certified, dairy-free |
Lowest declared potency in this guide Currently sold out Only 30 capsules Single strain — no broader coverage |
Follow the label, not the headline number. A capsule quoting 25 billion CFU and a powder quoting 100 billion CFU per gram are not comparable by volume, and a serving of the powder may be well under a gram.
Start low if you're moving up in strength. Higher doses are more likely to cause bloating and wind in the first fortnight. If you're switching from a 10 billion capsule to something substantially stronger, build up over two to three weeks rather than jumping. See can you take too many probiotics.
Timing: consistency beats precision — see when to take probiotics. Allow four weeks before judging: how long probiotics take to work. For the wider range, see best probiotics UK.
Probiotics are well tolerated by most people, but adjustment symptoms — bloating, wind, altered stools — are more common and more noticeable at higher doses. They usually settle within a fortnight. See probiotic side effects.
Speak to a doctor before taking a high-strength probiotic if you
Taking more than the label recommends does not accelerate results and makes side effects more likely. Further detail in probiotics safety.
On labelling: probiotics carry no authorised health claim in GB or the EU, so they are sold as food supplements and cannot legally be marketed as treating any condition.
See your GP — or seek urgent care — if you have:
Escalating the dose is not the answer to symptoms that aren't improving. Read more on when to see a GP about stomach symptoms and bowel cancer screening in the UK.
Welzo Ultra Purity Akkermansia at £21.99 — it declares 60 billion AFU per serving, it is the only high-potency option currently in stock, and it costs a fifth of the alternatives. Custom Probiotics B. Longum Powder declares a higher raw figure at 100 billion CFU per gram, but that is per gram rather than per serving and it is currently sold out. If evidence matters more than the number, Pure Encapsulations ProFloraGG at £20.47 gives you 25 billion CFU of the best-researched strain there is.
Not demonstrably. In the best dose-ranging trial in this field, 1×108 CFU improved IBS symptoms while a dose a hundred times larger did not beat placebo. Cochrane's effective range for antibiotic-associated diarrhoea is roughly 5 to 40 billion daily — not "as high as possible".
AFU counts cells by fluorescence rather than by growing colonies on a plate. It is often used for organisms that are difficult to culture conventionally, such as Akkermansia. The two units are related but not interchangeable, so don't compare them directly.
Not necessarily. If a 50 billion product contains ten strains, it may hold only 5 billion of the one you actually want. A single-strain product at its trial dose is often the more targeted purchase.
No — and they don't have them. Spore products are typically dosed at 1 to 5 billion because far more survives the stomach intact. Comparing spore CFU with freeze-dried CFU is misleading.
Exceeding the label dose won't speed anything up and makes bloating and wind more likely. In people who are immunocompromised or have indwelling lines, higher exposure carries a small additional risk — those groups should seek medical advice first.
During and after antibiotics, after a significant gut disruption, in multi-strain blends where each strain needs a meaningful share, or where a practitioner has specified it.
Some don't, which is a real limitation — Custom Probiotics Adult Formula CP-1 is an example. Without a figure you cannot compare potency or check it against a trial dose. Ask the manufacturer rather than assuming it must be high because the price is.
Not directly, but what matters more is whether the count is guaranteed at manufacture or at expiry. A large figure guaranteed only at the factory tells you little about the capsule you take months later.
Changing strain is usually more sensible than increasing dose. The evidence attaches to specific strains, not to CFU counts, so a different well-chosen organism is a better next step than more of the same.
Medical disclaimer: This article is for general information and education only and does not constitute medical advice, diagnosis or treatment. Food supplements are not a substitute for a varied and balanced diet, and are not intended to diagnose, treat, cure or prevent any disease. Potency figures are quoted exactly as declared on product listings at the time of writing; where no figure is declared we say so and have not estimated one. CFU and AFU are different units and are not directly comparable. Do not exceed the dose stated on the pack. Probiotics should be discussed with a doctor before use if you are immunocompromised, critically unwell, have an indwelling venous line, have inflammatory bowel disease, or are pregnant or breastfeeding; high-strength adult products are generally not appropriate for children. Prices, review counts and availability are correct at the time of writing and may change.
About the author
Dr Zeeshan Afzal (MBBS) is a practising medical doctor and Welzo's Medical Content Lead. He writes and reviews Welzo's health content to ensure it is accurate, evidence-based and aligned with current UK clinical guidance, translating complex research into practical advice patients can trust.
Valutazione eccellente da 26,523+ recensioni
Garanzia di rimborso al 100%
La nostra garanzia di rimborso ti assicura di ricevere i tuoi articoli o di ottenere un rimborso completo.
