What Happens If You Take Probiotics When You Don't Need Them?

Usually nothing you would notice. In the large reviews, side effects were no more common on live cultures than on placebo, the cultures were found in stool while taken and fell back towards baseline once dosing stopped, and no trial records a dependency. What some people get is a few days of wind or looser stools, and, in the respiratory trials, slightly fewer colds over a winter.^{1, 2, 3} Part of the Wellgard probiotics guide.

Evidence at a glance

• Trials: 23 randomised trials pooled in a 2022 Cochrane review, 6,950 people (respiratory infections); 36 trials, 9,312 adults, for pooled side effects.
• Largest single study: 117 people tracked through eight weeks of daily capsules and after stopping.
• Strongest measured outcome: at least one upper respiratory infection, risk ratio 0.76 after three months or more of daily use, low certainty; adverse events risk ratio 1.00 against placebo.
• Not tested: healthy adults beyond the months the respiratory trials ran; how long first-days wind or looser stools last.

Sources checked on 12 October 2026.

What happens when you take probiotics when you don't need them?

"Need" is not something a test measures: there is no test for a shortage of live cultures and no clinical definition of a "probiotic deficiency", so the question is what trials found when people without a diagnosed problem took them. The NHS page, last reviewed on 24 November 2022 and past its own review date, says that "for most people, probiotics appear to be safe".⁴ In the one tolerance trial built around the question, 17 healthy volunteers took 100 billion CFU a day of a single lactobacillus strain for three weeks, against 17 on placebo: gut symptoms and bowel habit were unchanged, liver and kidney blood tests showed no clinically relevant change, and the strain was found in the stool of 16 of the 17 people taking it while they took it.⁵ That pattern repeats across this page.

Should a healthy person take probiotics?

The clearest pooled finding in people without a condition is fewer respiratory infections after months of daily use. A 2022 Cochrane review of 23 randomised trials in 6,950 children, adults and older people found that those taking live cultures, mostly for more than three months, were less likely to have at least one upper respiratory infection (risk ratio 0.76 across 16 trials, 4,798 people; low-certainty evidence), less likely to have three or more (risk ratio 0.59, moderate certainty), had episodes about 1.2 days shorter, and were prescribed fewer antibiotics for them (risk ratio 0.58, moderate certainty).¹ That relative reduction is a few percentage points over a winter, and the authorised immune wording in Great Britain belongs to nutrients such as vitamins B12, C and D and zinc, not to any live culture. For everything else the evidence is in people with a condition, which is why this page points you to those pages. Whether a daily capsule suits you is the subject of should you take a probiotic every day?

Can taking a daily probiotic be harmful?

In pooled trials, no. Across 36 placebo-controlled trials in 9,312 adults taking live cultures alongside antibiotics, adverse events were no more common than on placebo (risk ratio 1.00); across the eight respiratory trials that reported them (2,456 people) the figure was 1.02; across 55 trials in more than 7,000 people with IBS the rate was not significantly higher.^{2, 1, 6} Where people did report something it was minor and mostly gut-related.¹

The exceptions are specific: rare bloodstream infections in people who are seriously ill. In a series of ten yeast bloodstream infections at one hospital, most patients were being fed by drip or tube or had a central line, six had been prescribed a yeast-based live-culture product, the three without those risk factors were aged 80 or over, and half died.⁷ So four groups ask their clinical team before starting: anyone with a weakened immune system (including chemotherapy or a transplant), anyone critically ill or in hospital with a central line, premature babies, and anyone whose doctor has said so, which in that series included the very old. The NHS says the same, that anyone with "an existing health condition or a weakened immune system" should talk to a doctor first.⁴ In pregnancy, a 2021 Cochrane review of seven trials in 1,647 women found more pre-eclampsia among those taking live cultures in gestational-diabetes trials (risk ratio 1.85, high-certainty evidence), so anyone pregnant or breastfeeding speaks to their midwife or GP first.⁸ What counts as too much is on side effects of too much probiotics.

Do you build up a tolerance or a dependency?

No trial records either. In a study that tracked one bifidobacterium strain in 117 people, 76 with IBS and 41 healthy, through eight weeks of daily capsules and afterwards, stool levels of the strain rose while it was taken and "declined and approached baseline once dosing ceased"; the authors call its presence transient.³ Live cultures pass through rather than settle, so a daily capsule is a daily habit rather than a one-off. The documented exception is one bifidobacterium strain that persisted for at least six months in 30% of the people who took it, without gut symptoms and without changing the rest of their resident community, and only in people who had little of that species to begin with.⁹

Why might you feel bloated or windy at first?

Because the usual mild reports in the trials are gut ones. The Cochrane respiratory review lists the adverse events it found as minor and mostly gastrointestinal, flatulence, diarrhoea and bowel pain among them, at a rate no different from placebo.¹ In the three-week healthy-volunteer trial at 100 billion CFU a day, symptom diaries, bowel frequency and stool consistency did not change at all.⁵ No trial has timed when early wind or looser stools begin or end, and none shows people getting worse before they get better, so "a few days" is the pattern of what people report rather than a measured figure; if symptoms last beyond a week or two, stop and ask a pharmacist or GP. More on can probiotics make you feel worse before better?

How would you know whether you needed one?

The trial, not a symptom checklist, is the test. The uses with pooled randomised evidence, from the most certain down:

• Diarrhoea during a course of antibiotics. Across 42 trials in 11,305 adults, people taking live cultures alongside their antibiotics were 37% less likely to get it (risk ratio 0.63, moderate certainty), the benefit clearest where baseline risk was moderate or high; the culture was started with the antibiotic, not afterwards.¹⁰
• IBS symptoms. Across 82 trials in 10,332 people, some strains and mixtures improved overall symptoms or pain, with moderate certainty for one group of strains and low to very low for almost everything else.⁶
• Constipation. Across 14 trials in 1,182 adults, live cultures shortened whole-gut transit by 12.4 hours and added 1.3 bowel movements a week, with bifidobacteria doing most of the work and a high risk of bias noted by the authors.¹¹
• Colds over a winter. The respiratory finding above, low to moderate certainty.¹

If none of those describes you, the respiratory trials and the safety record above are the evidence that applies. Tiredness, bloating and an irregular bowel are real symptoms with their own causes, each matched to its evidence on the signs people read as "needing" a probiotic.

What we don't know

• No trial has followed healthy adults taking live cultures for longer than the respiratory trials ran, so there is no long-term record in well people either way.
• No trial has timed when first-days wind or looser stools start or stop; the pooled reviews record only that they occurred no more often than on placebo.
• No trial has given live cultures to people chosen because they felt well and measured a gut-symptom benefit; the healthy-volunteer trials measured tolerance and recovery of the strain from stool.
• Case series cannot give a rate for the rare bloodstream infections in seriously ill people, so no one can say how rare "rare" is.

Frequently asked questions

Can long-term use of probiotics be harmful?

Most of the 23 trials in the Cochrane respiratory review ran for more than three months of daily use, and the eight that reported adverse events (2,456 people) recorded them at the placebo rate (risk ratio 1.02).¹ Beyond that there is no long-term trial in healthy adults, a gap rather than a warning; the groups with recorded harm are the four above.

Do probiotics make your gut lazy?

Nothing in the record says so. The strains are found in stool while taken and fall back towards baseline once dosing stops, and no trial has reported rebound, withdrawal or a gut that worked less well afterwards.^{3, 9}

Is it a waste to take them if I feel fine?

In people who felt fine, the measured result was fewer upper respiratory infections after three months or more of daily use (risk ratio 0.76) and fewer antibiotic prescriptions for them (risk ratio 0.58), at a side-effect rate no different from placebo.¹ That is a modest, low-to-moderate-certainty benefit rather than none.

What is the difference between a prebiotic and a probiotic?

A probiotic is the live organism; a prebiotic is the fibre the gut's own bacteria feed on, inulin and oligofructose being the best studied, and the expert definitions require a measured health benefit for both.¹² The Wellgard inulin guide explains what the prebiotic trials measured and the one wording the GB register allows; "prebiotic" itself is not an authorised claim.¹³

Sources

1. Zhao Y, Dong BR, Hao Q. Cochrane Database of Systematic Reviews, 2022 (acute upper respiratory tract infections). Cochrane review, 23 randomised trials, 6,950 people. PubMed 36001877.
2. Liao W, et al. Probiotics for the Prevention of Antibiotic-associated Diarrhea in Adults: A Meta-Analysis of Randomized Placebo-Controlled Trials. Journal of Clinical Gastroenterology, 2021. Pooled analysis of 36 trials, 9,312 adults. PubMed 33234881.
3. Charbonneau D, Gibb RD, Quigley EM. Gut Microbes, 2013 (stool recovery of an encapsulated bifidobacterium strain). Controlled tracking study, 117 people, 8 weeks plus follow-up. PubMed 23549409.
4. NHS. Probiotics. Page last reviewed 24 November 2022 (review overdue). nhs.uk/conditions/probiotics.
5. Wind RD, et al. British Journal of Nutrition, 2010 (tolerance and safety of a lactobacillus strain in healthy volunteers). Randomised trial, 34 healthy adults, 3 weeks plus 3 weeks' follow-up. PubMed 20691131.
6. Goodoory VC, et al. Efficacy of Probiotics in Irritable Bowel Syndrome: Systematic Review and Meta-analysis. Gastroenterology, 2023. Pooled analysis of 82 trials, 10,332 people. PubMed 37541528.
7. Poncelet A, et al. Saccharomyces cerevisiae fungemia: Risk factors, outcome and links with S. boulardii-containing probiotic administration. Infectious Diseases Now, 2021. Case series, 10 patients. PubMed 33934809.
8. Davidson SJ, et al. Cochrane Database of Systematic Reviews, 2021 (live cultures in gestational-diabetes trials). Cochrane review, 7 randomised trials, 1,647 pregnant women. PubMed 33870484.
9. Maldonado-Gómez MX, et al. Cell Host & Microbe, 2016 (persistence of a bifidobacterium strain in the human gut). Human feeding study, 6 months' follow-up. PubMed 27693307.
10. Goodman C, et al. Probiotics for the prevention of antibiotic-associated diarrhoea: a systematic review and meta-analysis. BMJ Open, 2021. Pooled analysis of 42 trials, 11,305 adults. PubMed 34385227.
11. Dimidi E, et al. The effect of probiotics on functional constipation in adults: a systematic review and meta-analysis of randomized controlled trials. American Journal of Clinical Nutrition, 2014. Pooled analysis of 14 trials, 1,182 adults. PubMed 25099542.
12. Gibson GR, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nature Reviews Gastroenterology and Hepatology, 2017. Expert consensus statement. PubMed 28611480.
13. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Statutory register. gov.uk.

Spotted something out of date? Email support@wellgard.co.uk with the page address and the sentence.