Probiotics are sold like a numbers game: 10 billion, 50 billion, 100 billion. The number on the front is colony-forming units, and it tells you how many live organisms the manufacturer expects at a certain point. It does not tell you which strains they are, whether they survive the trip, or whether they will do anything for you.
The evidence is strain-specific. A product that helps with one thing may do nothing for another. The category is lightly regulated in the United States, and "probiotic" has no legal definition that sets a minimum standard. Rules differ in the EU and elsewhere, and 'natural' does not mean risk-free.
This guide is about the products that publish enough for a skeptic to make a decision, and about the questions the front label avoids.
The short version#
Seed DS-01 is the most thoroughly documented multi-strain option, at a price. Ritual Synbiotic+ is simpler and shelf-stable. Culturelle is the single-strain LGG standby. Florastor is the yeast option. Thorne FloraMend Prime is a small, focused three-strain formula.
The short list#
Seed DS-01 Daily Synbiotic
Best for a broad formula with published testing
DS-01 is a 24-strain formula with 38.7 billion AFU per capsule. AFU, or active fluorescent units, counts live and active cells differently from CFU, so do not compare it directly with a 50-billion-CFU product. The capsule is delayed-release, and the company publishes allergen and glyphosate testing.
Seed cites published research on its strains, which is not the same as proof that the finished product works. The subscription is the main way to buy it, and the price is high. It does not carry NSF Certified for Sport.
Buy it if you want a broad, well-tested formula and do not mind the price. Do not buy it expecting a remedy for a digestive condition.
What works
- 24 strains
- 38.7 billion AFU per capsule
- Delayed-release capsule
- Tested for allergens and glyphosate
What doesn't
- Expensive
- Subscription only
- AFU is not directly comparable to CFU
- No sport certification
Ritual Synbiotic+
Best for a shelf-stable daily capsule
Ritual's synbiotic combines 11 billion CFU of LGG and BB-12 with a prebiotic and a postbiotic, tributyrin. It is shelf-stable, one capsule a day, and the capsule is delayed-release. The ingredient list is short.
Two strains is narrow, and 11 billion CFU is modest. The subscription and the price per capsule are the other drawbacks. It does not carry a sport certification.
Buy it if you want a simple, shelf-stable capsule. Do not buy it if you want a broad formula.
What works
- 11 billion CFU
- LGG and BB-12 strains
- Includes a prebiotic and postbiotic
- Shelf-stable
What doesn't
- Only two strains
- Modest CFU count
- Subscription model
- No sport certification
Culturelle Digestive Daily
Best for a single well-studied strain
Culturelle's Daily is built around LGG, one of the most studied probiotic strains. It lists 10 billion CFU through expiration, which is the number that matters; a count at manufacture is less useful. The capsule also contains inulin, a prebiotic fiber.
Some versions include titanium dioxide, sucrose, or maltodextrin, so read the other-ingredients line. It is a single strain, and the evidence for LGG does not transfer automatically. There is no sport certification.
Buy it if you want a cheap, widely available single strain. Do not buy it if you want a broad formula.
What works
- LGG strain
- 10 billion CFU through expiry
- Inexpensive
- Easy to find
What doesn't
- Single strain
- Some versions contain additives
- No sport certification
- Evidence is strain-specific
Florastor
Best for people who specifically want S. boulardii
Florastor is not a bacterial probiotic. It is Saccharomyces boulardii CNCM I-745, a yeast, which means antibacterial antibiotics do not kill it. That is why it is often used alongside antibiotics, a decision that belongs with a clinician rather than a label. It is shelf-stable and does not need refrigeration.
The dose is 250 mg, roughly 5 billion CFU, low compared with bacterial products but not directly comparable. It is a single strain, and people who are immunocompromised should talk to a clinician before using any live product, yeast included. The price is high for a five-billion-count capsule.
Buy it if you specifically want S. boulardii. Do not buy it as a generic probiotic.
What works
- Saccharomyces boulardii CNCM I-745
- Not killed by antibacterial antibiotics
- Shelf-stable
- No refrigeration
What doesn't
- Single strain
- About 5 billion CFU
- Immunocompromised caution
- Expensive per capsule
Thorne FloraMend Prime Probiotic
Best for a short strain list from a careful manufacturer
FloraMend Prime uses L. gasseri KS-13, B. longum MM-2, and B. bifidum G9-1 at 5 billion CFU. It is shelf-stable. Thorne's quality documentation is better than most.
The count is low, the strain list is short, and the price is high. It does not carry NSF Certified for Sport, despite Thorne's other products often doing so. If you want a broad formula, this is not it.
Buy it if you want a small, focused formula. Do not buy it if you are counting billions.
What works
- Three documented strains
- Shelf-stable
- Good manufacturing disclosure
- Simple capsule
What doesn't
- 5 billion CFU
- Only three strains
- Expensive
- No sport certification
What the CFU number leaves out#
Strain, not just species. Lactobacillus rhamnosus is a species. LGG is a specific strain with its own research. A bottle that lists the species but not the strain code is asking you to trust a category, not a product.
CFU at manufacture versus at expiry. A label can say 50 billion CFU and mean at the time it was made. By the time it sits in your cupboard for six months, the number may be much lower. A count through expiry is the more honest claim.
CFU and AFU are different rulers. AFU counts active cells with a fluorescent method; CFU counts cells that can form colonies. A 38.7 billion AFU product may be comparable to a much larger CFU number, or not. Do not compare the two as if they were the same unit.
Survival is the whole point. A delayed-release capsule, a shelf-stable formulation, and directions that avoid hot drinks all help. A probiotic that dies in the stomach is an expensive way to feed your stomach acid.
Prebiotics and postbiotics are not probiotics. A prebiotic is food for microbes, usually a fiber. A postbiotic is a metabolite or inactivated cell. A synbiotic combines them. None of these words promises an effect.
Testing seals are uneven in this category. NSF Certified for Sport and USP Verified are meaningful, but many probiotic brands rely on their own quality programs. GMP is a manufacturing standard, not a check of what is in the capsule. If you are immunocompromised, pregnant, or seriously ill, ask a clinician before starting any live product.
Questions people actually ask#
Do probiotics actually colonize your gut?#
Most commercial strains pass through rather than take up permanent residence. That does not make them useless; the effects may come from what they do while they are there. But "colonize" is often used more loosely than the evidence supports.
Should I take a probiotic with antibiotics?#
Some clinicians recommend specific strains, such as S. boulardii or LGG, in certain situations. Timing and strain matter, and it is not a universal rule. Ask the prescriber or a pharmacist rather than guessing from a shelf label.
Are refrigerated probiotics better?#
Not automatically. A shelf-stable product with a count through expiry can be a better choice than a refrigerated one that sat in a warm delivery van. Follow the storage instructions and check the date.
A probiotic is a specific tool, not a daily virtue. Start with the strain and the reason, then check the count at expiry and who tested it. The billions are the easiest number to print and the least useful.
