Probiotic Safety: Immunocompromised People, Infants, Serious Illness and Fiber Cautions

Urgent first: premature babies and very sick people

Do not give a probiotic to a premature baby on your own. The US FDA (October 2023) said preterm infants given a probiotic product are at risk of invasive, potentially fatal infection caused by the bacteria or yeast in it, and that it had not approved any probiotic as a drug or biological product for infants of any age. [1] Some hospital teams use specific products under supervision; that is a decision for the baby's neonatal team only. See Probiotics in preterm infants and necrotizing enterocolitis.

People who are immunocompromised, critically ill or have a central line should not start a probiotic without their care team's approval. NCCIH (US) says the risk of harmful effects is greater in people with severe illnesses or compromised immune systems. [2] The NHS (UK) says to talk to a doctor before any probiotic supplement if you have an existing health condition or a weakened immune system. [3] Details: Probiotics and bloodstream infection.

If someone becomes feverish, has chills or seems seriously unwell after starting a probiotic, get medical care promptly. In the US, call 911 for an emergency.

The overall safety picture

Probiotics have an extensive history of apparently safe use, particularly in healthy people, but few studies looked at safety in detail, so how often side effects happen and how severe they are is not well known. [2] A 2015 review noted safety outcomes are inconsistently reported in clinical trials. [4] A 2026 review concluded safety depends on strain, indication and patient, and that rare infectious complications have been reported in high-risk patients. [5] Safety findings for one strain do not automatically apply to another; see why one strain's result does not transfer.

This page uses simple evidence labels: established risk (stated by a health body), limited evidence (case reports or small studies) and Proper evidence not found.

Possible harms

NCCIH lists infections, harmful substances made by the microorganisms, and transfer of antibiotic resistance genes to other microorganisms in the gut. [2] In trials of probiotics to prevent C. difficile diarrhea, the most common events in both treated and comparison groups were cramping, nausea, fever, soft stools, gas and taste changes. [9] Everyday reactions are covered in Probiotic side effects.

A 2026 meta-analysis of long-term (12 weeks or more) trials in at-risk groups found no increase in reported adverse events, but concluded the evidence does not permit conclusions about long-term safety because reporting was inconsistent. [10]

Product quality

Some probiotic products have been reported to contain microorganisms other than those on the label. [2] In one laboratory study of products sold in one market, almost none met the label claim and half contained antibiotic-resistant strains. [11] That is one study, not a measure of every market. The AGA technical review notes manufacturing contaminants, including a fungal contaminant linked to a fatal case in a preterm infant. [12] The NHS notes probiotics are generally classed as food rather than medicine. [3] See Product quality and contamination, How supplements are regulated and Quality testing and certifications.

Fiber and prebiotic cautions

Fiber products are not probiotics, but are often sold alongside them.

• Add fiber slowly. NIDDK advises adding fiber a little at a time so the body gets used to the change. [13]
• Gas. Undigested carbohydrates reach the large intestine, where bacteria break them down and make gas. [14]
• Fluids (psyllium). MedlinePlus says you must drink at least 8 ounces (240 milliliters) of liquid with psyllium. [15]
• Medicine timing. MedlinePlus lists some medicines (digoxin, aspirin and nitrofurantoin) that should not be taken within 3 hours of psyllium. [15] A pharmacist can check your own list.
• Conditions. Tell a clinician about intestinal blockage, rectal bleeding or difficulty swallowing before using psyllium. [15]
• Powder. Avoid breathing in psyllium powder when mixing it. [15]

More: Psyllium and fiber supplements.

Other points

• Sweeteners and added ingredients. The NHS advises people with IBS to avoid products containing the sweetener sorbitol. [16] Read the full ingredient list: Reading supplement labels.
• Talk to a clinician or pharmacist first. NIDDK advises talking with your doctor before using supplements, probiotics or complementary approaches for gas. [17]
• Report problems. See Side effects, allergic reactions and reporting and Medication interactions.
• Never stop or change a prescribed medicine because of anything on this page. Speak to the prescriber.