Probiotics

Are probiotics safe?

For healthy adults, yes. Probiotics are live bacteria or yeasts, mostly Lactobacillus (now split into several genera), Bifidobacterium and Saccharomyces boulardii, and most people who take them get nothing worse than a few days of wind and bloating. NCCIH, part of the US National Institutes of Health, describes an extensive history of apparently safe use in healthy people.

The picture changes for people whose defences are down. Because the product is alive, it can invade the bloodstream of a premature baby or a patient in intensive care. In 2023 the FDA linked one infant death to a probiotic used in hospitals and warned against giving these products to preterm infants. The second problem is what is in the capsule: independent tests regularly find fewer live organisms than the label claims, or different species.

Who should not take probiotics?

Three groups carry real risk.

Premature babies. On 29 September 2023 the FDA told doctors that preterm infants given probiotics are at risk of severe, potentially fatal infections. A baby treated with Evivo with MCT Oil, which contains Bifidobacterium longum subsp. infantis, died of sepsis with a matching strain. The FDA said it knew of more than two dozen other adverse events in the US since 2018. Earlier, in October 2014, a Connecticut baby born at 29 weeks died of gastrointestinal mucormycosis after seven days of ABC Dophilus Powder, a supplement later found to be contaminated with mould (CDC report, 2015).

People in intensive care or with a central line. A 2019 study in Nature Medicine found a much higher rate of Lactobacillus bloodstream infection in ICU patients given probiotics, and whole-genome sequencing showed the bacteria in their blood came from the capsules. Yeast probiotics carry a similar risk of fungal bloodstream infection when capsules are opened near catheters.

People with severe acute pancreatitis. In the Dutch PROPATRIA trial (The Lancet, 2008), 298 patients were given a multispecies probiotic or placebo through a feeding tube. Infections did not fall, but 24 of 152 patients on probiotics died (16%) against 9 of 144 on placebo (6%). Nine on probiotics developed bowel ischaemia, eight of them fatal, and none on placebo did.

People with weak immune systems from chemotherapy, transplant drugs or advanced HIV should ask their doctor before using any live product.

Do probiotics interact with medicines?

Not in the way herbs such as St John's wort do. Probiotics do not change how the liver handles drugs. The interactions run the other way:

• Antibiotics such as amoxicillin can kill bacterial strains, which is why people often take the probiotic a few hours after the antibiotic dose.
• Antifungal drugs kill Saccharomyces boulardii.
• Immunosuppressants and cancer treatment do not interact chemically, but they make it easier for the probiotic to cause an infection.

Is the label accurate?

Often not. A 2016 study in Frontiers in Microbiology tested 52 commercial probiotic supplements with methods published by the US Pharmacopeia. Seventeen (33%) held fewer live organisms than the label claimed before their expiry date, and only 30 (58%) contained exactly the species listed; the rest had missing, wrongly named or unlisted species. Smaller European studies have found similar gaps, including products with no live bacteria at all.

Probiotics are a hard product to make well. Counts fall with heat, moisture and time, so a capsule that met its claim at the factory may not do so a year later on a warm shelf. NCCIH warns that some products have contained microbes not on the label, and some of these may pose serious health risks.

In the US, probiotics sold as supplements do not need FDA approval before sale. A product marketed to treat or prevent a disease is a drug and needs approval, which is why the FDA sent warning letters in 2023 to Infinant Health and to Abbott, whose Similac Probiotic Tri-Blend was sold to hospitals. Abbott stopped selling it.

How to use probiotics more safely

1. Pick a named strain, not just a species. Trials test strains such as Lactobacillus rhamnosus GG or S. boulardii CNCM I-745, and the label should list them.
2. Look for a count "at expiry", not "at manufacture", and follow the storage instructions. Some products need the fridge.
3. Prefer a third-party seal such as USP VerifiedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary or NSF, which check identity and count.
4. Stop and see a doctor if you get a fever, chills or worsening stomach pain while taking one, especially with a weak immune system.
5. Do not give probiotics to a premature or sick newborn unless the neonatal team decides to.

How we scored it

Interactions (85) and dose safety (80) are strong because healthy adults rarely come to harm. Organ toxicity (78) reflects bloodstream infections and the pancreatitis deaths in vulnerable patients rather than liver or kidney injury. Product quality (60) is the weakest factor: a third of products miss their count. Regulatory actions (70) reflect the FDA's 2023 preterm warning. For other everyday supplements with clean records, compare magnesium and fish oil.