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: ~9 minutes
A product with ten researched strains sounds ten times better than one with a single strain. It is not — and the reason why is the most useful thing to understand about this entire category.
Table of contents
If you have a specific complaint and there's a strain trialled for it, buy that strain. Single-strain products let you match the organism and dose to the research.
If you don't know what you need and just want general support, a multi-strain blend is a reasonable hedge — provided it discloses what's in it.
What you should not do is assume more strains means more effective. That intuition is wrong, for two separate reasons.
Probiotic effects are strain-specific. This is the foundational principle of the field, set out in the ISAPP consensus definition, and it has a consequence most product marketing quietly ignores.
Evidence attaches to a specific organism at a specific dose. It does not transfer to a different strain of the same species, and it does not transfer to a blend just because that blend contains the strain.
This is why a blend containing Lactobacillus rhamnosus GG does not inherit LGG's several hundred trials. It contains an organism that has been studied; it is not itself a studied product. See Lactobacillus rhamnosus GG.
The second issue is arithmetic, and it's easy to check.
A product advertising 50 billion CFU across 10 strains averages 5 billion of each. If the trial behind the strain you actually care about used 10 billion, you are taking half the studied dose — inside a product whose headline number looks impressive.
Worse, most blends use proprietary blends that don't disclose the split. The 50 billion could be 45 billion of one cheap organism and slivers of the other nine. You cannot tell, and the label is designed so you can't.
And higher totals are not the fix. In the best dose-ranging trial in this field, a B. longum strain improved IBS symptoms at 1×108 CFU while a dose one hundred times larger did not beat placebo. More is not automatically better — see CFU vs AFU and how to choose a probiotic.
The counter-argument is genuinely reasonable and deserves a fair hearing.
The practical test: does the manufacturer disclose the per-strain amounts, and has this specific formulation been studied? If yes to both, a multi-strain product is a strong choice. If no to both, you're buying a longer ingredient list rather than more evidence.
| Feature | Single-strain | Multi-strain |
|---|---|---|
| Matches trial evidence | Directly | Only if the blend was tested |
| Dose transparency | Clear | Often hidden in a blend |
| If you react badly | You know the culprit | You don't |
| Breadth of coverage | Narrow by design | Broader |
| Typical cost | Lower | Higher |
| Best when | You have a target | You don't |
Pure Encapsulations ProFloraGG 25B — £20.47 · 138 reviews · Available from Welzo. The clearest example of the single-strain case: the named GG strain at 25 billion CFU, plus a carrier and an HPMC capsule. Three ingredients, nothing to hide. Stock is intermittent.
Swanson L. plantarum (pictured) — £5.93 · 132 reviews · Available from Welzo. Around 20p a day in a delayed-release capsule. It names the species rather than the strain, so no specific trial evidence attaches — but as a cheap way to trial one organism at a time, it does the job.
Klaire Labs Ther-Biotic Complete — £50.63 (from £59.00) · 94 reviews · Available from Welzo. A practitioner-facing multi-strain formula combining Lactobacillus and Bifidobacterium species, which is the sensible rationale for a blend — covering both the small intestine and the colon.
Custom Probiotics Adult Formula CP-1 (pictured) — £77.41 · 152 reviews · Available from Welzo. A 90-capsule acidophilus-and-bifidus formula — a three-month supply, and the most-reviewed product in this guide. Note that the listing does not declare a CFU figure, which is exactly the transparency limitation described above; check the pack before buying if the number matters to you.
Both are well tolerated by most people. Wind, bloating and altered stools in the first week or two are common and usually settle. If you're prone to reacting, a single-strain product taken at a low starting dose is the more sensible route — see probiotic side effects.
Speak to a doctor before taking any probiotic if you
These cautions apply regardless of strain count. More 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:
Adding more strains is not the answer to symptoms that aren't improving. See when to see a GP and bowel cancer screening.
Not automatically. Probiotic effects are strain-specific, so a blend of ten researched strains isn't ten times as evidenced — it's an untested combination unless that specific blend was studied. Single-strain products let you match the research directly.
They can interact — competing for substrate or producing compounds that inhibit one another. Good manufacturers test their formulations for compatibility. It's another reason a blend needs testing as a product rather than being assumed to work.
There's no correct number, and treating it as a spec to maximise is the mistake. One well-matched strain at its trial dose beats twelve at unknown doses.
Most use proprietary blends, which are legal and commonplace but mean you can't verify the split. A headline of 50 billion could be dominated by one cheap organism. Products that disclose per-strain amounts deserve preference.
No. In the best dose-ranging trial in this field, a strain worked at one dose and failed at a dose one hundred times higher. Matching the studied dose matters more than maximising the number.
Usually single-strain, matched to your symptom pattern, since the IBS evidence is strain-specific. Get IBS properly diagnosed before self-treating.
Single-strain, using an organism trialled for that purpose. This is one of the clearest cases where matching the research beats broad coverage.
Switch to single-strain and start low. With a blend you can't tell which organism caused it; with one strain you can identify and avoid it.
Yes, and it gives you dose control a blend can't. Introduce them a few weeks apart so you can tell what each is doing.
No — single or multi-strain. They're transient and pass through over days. Products promising to permanently "restore" your microbiome overstate what any probiotic does.
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. Probiotic effects are strain-specific: evidence for one strain does not transfer to other strains, to other species, or to blends containing that strain unless the blend itself has been studied. 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, and supplements for children should be discussed with a GP or health visitor. 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.
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