What to Look for in a Probiotic — Strains, CFU, and Red Flags

The probiotic market is overwhelming. Thousands of products, dozens of strains, CFU counts ranging from 1 billion to 100 billion, and marketing claims that range from plausible to absurd. Most “best probiotics” lists won’t tell you this, but choosing a probiotic is fundamentally different from choosing most other supplements — because strain specificity matters more than almost anything else.

The Single Most Important Thing: Strain Specificity

Here’s what most people get wrong about probiotics: they shop by genus and species (like Lactobacillus acidophilus) when they should be shopping by strain.

A probiotic strain is identified by three parts: Lactobacillus rhamnosus GG. That last part — the strain designation — is what determines whether the probiotic has clinical evidence behind it. L. rhamnosus GG has hundreds of clinical trials. A different strain of L. rhamnosus may have zero.

If a product doesn’t list specific strain designations, you have no way to evaluate its evidence base. This is the #1 red flag in probiotic shopping.

CFU Count: More Isn’t Always Better

CFU (Colony Forming Units) measures the number of viable organisms per dose. The marketing arms race has pushed CFU counts to absurd levels — 100 billion, 200 billion. But clinical trials typically use 1–20 billion CFU and show clear benefits at these doses.

What matters more than total CFU:

• CFU at expiration, not at manufacture: Probiotics die over time. A product that guarantees “50 billion at time of manufacture” might deliver 5 billion by expiration. Look for guaranteed CFU through expiration date.
• CFU per strain, not per total blend: A product with “50 billion CFU from 30 strains” may have meaningful doses of none of them. Clinical studies typically use 1–10 billion of a single specific strain.
• Survival to the gut: A high CFU count means nothing if the organisms die in stomach acid. Delivery mechanism matters (see below).

Clinically Studied Strains Worth Knowing

These strains have significant clinical trial evidence behind them:

• Lactobacillus rhamnosus GG: One of the most-studied probiotic strains in the world. Evidence for antibiotic-associated diarrhea prevention, IBS symptom reduction, and immune support.
• Saccharomyces boulardii: A yeast-based probiotic (not a bacterium) with strong evidence for preventing and treating diarrhea, including C. difficile-associated diarrhea. Unique in being naturally resistant to antibiotics.
• Bifidobacterium lactis BB-12: Evidence for regularity, immune function, and gut barrier integrity. One of the best-documented bifidobacteria.
• Lactobacillus plantarum 299v (LP299V): Specifically studied for IBS, particularly bloating and abdominal pain.
• VSL#3 combination: A specific multi-strain formulation with evidence for ulcerative colitis and IBS. The exact strains and ratios matter — generic copies may not have the same evidence base.

The key takeaway: match the strain to your goal. A probiotic for general gut health, IBS, immune support, or antibiotic recovery should ideally use strains studied for that specific purpose.

Delivery and Survival

Probiotics must survive stomach acid to reach the intestines alive. Several factors affect this:

• Enteric-coated or delayed-release capsules: Resist stomach acid and dissolve in the intestines. This significantly improves survival rates.
• Spore-based probiotics: Bacillus strains naturally form protective spores that survive stomach acid, heat, and shelf storage. No refrigeration needed.
• Taking with food: Eating before or during probiotic consumption buffers stomach pH and may improve survival. Some studies suggest taking probiotics with a meal containing fat is optimal.
• Refrigeration: Many traditional Lactobacillus and Bifidobacterium strains are sensitive to heat and moisture. Shelf-stable claims should be backed by stability testing data.

Red Flags When Shopping

• No strain designations listed: Just saying “Lactobacillus acidophilus” without a strain code means you can’t verify any clinical evidence.
• “30+ strains!” as a selling point: More strains doesn’t mean more effective. Many of those strains may have no individual evidence and may compete with each other.
• CFU guaranteed only at manufacture: Should be guaranteed through the expiration date.
• Proprietary blend with undisclosed strain amounts: You need to know how much of each strain you’re getting.
• Broad health claims without citing specific strains: “Supports immune health, digestive wellness, mood, weight management, and skin health” from one product is marketing, not science.

What to Actually Look For

A quality probiotic should have:

• Specific strain designations (e.g., L. rhamnosus GG, not just L. rhamnosus)
• Clinical evidence for those specific strains at the doses provided
• CFU guaranteed through expiration date
• Individual CFU counts per strain, not just a total
• Appropriate delivery mechanism (enteric-coated, spore-based, or with clear survival data)
• Third-party testing for purity and potency
• Clear storage instructions

The Bottom Line

Probiotics are not interchangeable. A high CFU count and a long strain list mean nothing without clinical evidence for the specific strains at the specific doses in the product. Start by identifying what you want the probiotic to do (digestive support, immune health, IBS relief, antibiotic recovery), then find a product using clinically studied strains for that purpose. Ignore the CFU arms race and focus on evidence, strain specificity, and delivery mechanism.

Browse probiotic supplements in the Formulate catalog →