Probiotics
Lactobacillus, Bifidobacterium, Lactobacillus rhamnosus GG, LGG, Lactobacillus casei Shirota, Yakult, Saccharomyces boulardii, Enterol
Forms: Capsules, powders, drinks and fermented foods with one or several strains of bacteria or yeast; synbiotics add fibre that feeds them. A few strains are registered medicines in some countries.
Live bacteria or yeasts taken as supplements or in fermented foods. Benefits depend on the strain; the best athlete data concern fewer upper-respiratory infections.
Legal status: Sold freely as supplements and foods; some strains are registered medicines (varies by country); not on the WADA Prohibited List.
What it gives
What people use it for and what the sources report. Strength is a general category from the literature: mild, clear or strong.
Purpose
Athletes take them against colds during heavy training and travel, for gut comfort and after antibiotics; on GLP-1 drugs a consensus lists them for persistent diarrhea. Sports bodies see modest, strain-specific benefits.
What people seek and what studies show
IOC: moderate support in athletes; a Cochrane review (12 studies, 3,720 people) found about half as many upper-respiratory episodes, about 2 days shorter. For exercise gut symptoms reviews disagree: modest relief in a few studies, but no effect in 4 of 5 studies in a 2025 position statement. Claims for body composition or testosterone rest on early research.
Effects
Evidence: Limited human data — some human studies, small or short
Possible side effects
Possible does not mean certain: whether they appear depends on dose, duration, individual response and monitoring. Below — what to watch for and how to notice it early.
3 possible side effects
level · how often it is reportedWhat the levels mean
Level: general category from the literature, not your personal risk
How often
- common
- reported often
- possible
- reported in some people
- rare
- reported occasionally
How to lower the risks
For 2 of 3 possible side effects there are specific ways to lower the chance: everyday measures, supplements and options a doctor may consider. For the rest — general advice. No measure removes a risk completely.
Low-level side effects
How each option helps (13)
- Avoiding alcohol and other liver stressors — Alcohol irritates the stomach, adds to aspirin-related stomach bleeding and is a main cause of pancreatitis; avoiding it lowers that part of the risk, not drug nausea.
- Fluid & electrolyte management — Dehydration makes gut complaints in long or hot sessions more likely; drinking enough helps that part, not nausea from drugs or pancreatitis.
- Working with a doctor — Assesses persistent vomiting or severe belly pain (possible pancreatitis or gallstones), can add stomach protection to aspirin and adjusts GLP-1 treatment when nausea persists.
- Omeprazole and other proton-pump inhibitors (PPIs) — A doctor may consider it for heartburn, reflux, dyspepsia or ulcer cover with aspirin or NSAIDs (label, guidelines). It does little for nausea or diarrhea; long-term use has its own risks.
- Famotidine (H2 blockers) — Relieves and prevents occasional heartburn (label); it does not help nausea or diarrhea. For reflux damage or long-term ulcer cover a doctor usually turns to a PPI.
- Loperamide — Eases acute diarrhea, including on GLP-1 drugs (consensus), but not its cause; not for nausea or bloody stools. Fluids and salts still come first.
- Macrogol (polyethylene glycol laxative) — Eases constipation, including on GLP-1 drugs, with strong guideline backing in chronic constipation; not for nausea or heartburn. Rectal bleeding or worsening pain needs a doctor.
- Magnesium carbonate — An antacid for heartburn and indigestion (label), often combined with other antacids; it does not help nausea and may itself loosen stools. Not with weak kidneys.
- Magnesium citrate — In larger amounts a saline laxative for occasional constipation (label); not for nausea. It causes cramps and loose stools itself; ask a doctor first with kidney disease.
- Magnesium hydroxide — Labeled as an antacid for heartburn and a laxative for occasional constipation; short-term relief only, not for nausea. It can itself cause diarrhea; not with weak kidneys.
- Magnesium oxide — An osmotic laxative that eases constipation (conditional guideline advice); not for nausea or heartburn. It loosens stools itself; with weak kidneys blood magnesium can rise.
- Pancreatin + dimeticone (Pankreoflat) — Registered for heaviness and bloating after meals; it does not ease drug nausea, heartburn or aspirin stomach pain. Not with pork or cow's milk protein allergy or acute pancreatitis.
- Psyllium husk fiber — Soluble fiber eases constipation, including on GLP-1 drugs, and can firm loose stools; it does not help nausea or heartburn and can itself cause bloating and gas.
How each option helps (7)
- Hepatitis B vaccination — Very effective, but only against hepatitis B — not hepatitis C, HIV, abscesses or immunity lowered by corticosteroids. A blood test can show past infection or immunity; it need not delay vaccination.
- Sleep hygiene — Short sleepers caught colds more often in a virus-exposure study, and IOC advice on illness in athletes includes enough sleep; it does not offset corticosteroids.
- Sterile injecting equipment & needle-and-syringe services — Acts only on infections that come with injecting (skin infections, blood-borne viruses); it does nothing for the lowered immunity from corticosteroids, the main cause of this risk.
- Working with a doctor — A doctor can bring vaccinations up to date, treat infections early and keep in mind that fever can be masked on corticosteroids; their immune suppression stays.
- Vitamin C (ascorbic acid) — Under heavy physical stress (marathon runners, soldiers) regular intake roughly halved colds in trials; no effect in most people, on wound healing or on corticosteroid effects.
- Vitamin D3 — A large analysis of trials found slightly fewer respiratory infections; the benefit is small and it does not offset corticosteroid immune suppression. A blood test shows if you are low.
- Zinc — Zinc acetate lozenges may shorten a cold (IOC: moderate support); they do not make colds less frequent, and too much zinc weakens immunity. Corticosteroid effects stay.
Check-ups and blood tests
Which tests and check-ups to do and when. This is a guide — agree the exact schedule with a sports physician.
Myth & fact
“Any probiotic boosts immunity and fixes the gut.”
Benefits are strain- and dose-specific (ISSN). The IOC sees moderate support only for fewer respiratory infections, and a 2025 sports position statement found no effect on exercise gut symptoms in most studies.
Combinations
No specific combination notes for this substance yet. The stack check still adds up side effects that several substances share.
Sources
Possible side effects by body system, combinations, how to lower the risks and which tests to do — free.
Open the stack check