Can Probiotics Make You Tired?

No trial or pooled analysis records tiredness as a side effect of live cultures: across 36 randomised trials in 9,312 adults, adverse events on live cultures matched placebo (relative risk 1.00),¹ and the 2025 Cochrane count across 37 trials in 11,911 people lists abdominal cramping, nausea, fever, soft stools, flatulence and taste disturbance as the common events, not drowsiness or fatigue.² Nor do live cultures give energy: no trial in healthy adults has measured it. This page sets out the side effects the trials do record, what changes in the first weeks, and what the two small trials in people with chronic fatigue syndrome measured.

The gut holds trillions of bacteria, a mixture whose balance shifts with diet, illness and antibiotics, and the symptoms people notice most are bloating and wind. Live cultures add specific species to that mixture, and their trial record is by outcome: diarrhoea during antibiotics, constipation, colds and mood have each been measured. If you are taking them with tiredness in mind, the trials below are the ones to read. For what probiotics are and how they are tested, see what are probiotics.

What are the negative side effects of taking probiotics?

For most people, none that last. The NHS says that for people with a healthy immune system probiotics "shouldn't cause any unpleasant side effects", and that anyone with an existing health condition or a weakened immune system should talk to a doctor before taking them.³ The events the pooled trials record are gut events in the first days: wind, bloating, cramping and a softer stool, at rates no higher than placebo.^{1, 2} For heartburn, see can probiotics cause acid reflux? Three situations deserve more care.

1. Digestive discomfort that does not settle

A new culture can bring cramps, bloating, wind or looser stools for a few days. If they have not settled within a week or two, or are getting worse, stop taking the capsules and see a GP.

2. Allergy or intolerance

Capsules and drinks can contain milk, soya, gluten or egg from the culture medium or the carrier. If you have an allergy or intolerance, read the ingredients list before you start, and if you react, stop and speak to a GP or pharmacist.

3. Small intestinal bacterial overgrowth (SIBO)

The small and large intestine carry different bacteria, and SIBO is the name for bacteria multiplying in the small intestine, where they cause bloating, cramps and wind. One US case series in 2018 described 30 people with gas, bloating and "brain fogginess", all of whom were taking live cultures; 68% had SIBO against 28% of eight similar patients without brain fog, and after stopping the cultures and a course of antibiotics the brain fog resolved and gut symptoms improved in 23 of 30.⁴ Three researchers with declared industry interests replied that the cultures had not been shown to be the cause.⁵ It is a single-centre report, not a trial. If bloating, cramps and wind persist, stop the capsules and ask a GP.

What are the symptoms of too much probiotics?

The same gut symptoms, not tiredness. Bloating, wind, cramps and a softer stool are what the trials record; they are usually early and usually pass within a week or two.^{1, 2} If they persist, stop the capsules. The full record, including the one trial that tested 100 billion CFU a day, is on the side effects of too much probiotics.

What happens to your body when you start taking probiotics?

The changes the trials have measured are these.

• Bowel habit. Across 14 randomised trials in 1,182 adults with functional constipation, live cultures increased stool frequency by 1.3 bowel movements a week and shortened gut transit time by 12.4 hours; the effect was significant for Bifidobacterium lactis and not for Lactobacillus casei Shirota, and the authors note high heterogeneity and risk of bias.⁶
• Colds. In a Cochrane review of 12 trials in 3,720 children, adults and older people, fewer people taking live cultures had at least one cold or similar upper respiratory infection (odds ratio 0.53), on low-quality evidence.⁷
• Bloating in the first days. The common early event, at rates no higher than placebo in the pooled trials.^{1, 2}
• Mood. Across 34 controlled trials pooled in 2019, live cultures had small effects on depression scores (d = -0.24) and anxiety scores (d = -0.10), larger in clinical samples than in community ones.⁸ The mood evidence is on can probiotics make you anxious.

None of these outcomes is energy. For what the trials have measured in women specifically, see what do probiotics do for women?

Can you take probiotics long term?

The pooled safety counts above come from courses of a few weeks, and individual trials have run from days to a year or more. The NHS says that for most people probiotics appear to be safe, and that anyone with an existing health condition or a weakened immune system should talk to a doctor first.³ If you tolerate a culture, there is no trial evidence against taking it daily for as long as you want the measured effect; how long the trials ran for each outcome is on how long should I take probiotics. If you take prescription medicine, check with a pharmacist that the two can be taken together.

Frequently asked questions

Do probiotics give you energy?

No trial in healthy adults has measured energy, and no live culture carries an authorised energy claim in Great Britain; the authorised "reduction of tiredness and fatigue" wording belongs to nutrients such as vitamin B12, vitamin C, iron and magnesium. The two small trials in chronic fatigue syndrome measured different things: in 15 patients taking a three-strain product for four weeks there were "no significant changes in fatigue and physical activity scores", and in 39 patients taking a single strain for two months anxiety scores fell (P = 0.01) while fatigue was not the outcome.^{9, 10}

Can probiotics make you sleepy?

Sleepiness is not an adverse event the pooled trials record. Across 36 randomised trials in 9,312 adults the adverse-event rate on live cultures matched placebo (relative risk 1.00), and the 2025 Cochrane count across 37 trials lists abdominal cramping, nausea, fever, soft stools, flatulence and taste disturbance as the common events, not drowsiness.^{1, 2} The one pooled analysis of sleep, 12 trials in 789 people, measured sleep quality at night and found it slightly better with daily use (standardised effect -0.39, high heterogeneity); no trial has measured daytime sleepiness.¹¹ Persistent unexplained tiredness is a GP matter.

Should I stop probiotics if I feel tired?

Stopping for a week or two is a fair test. If the tiredness continues once any gut symptoms have settled, it has another cause, and a GP can check the common ones. Tiredness that has lasted weeks is worth that appointment whether or not you take a supplement.

Sources checked on 19 October 2026.

Sources

1. 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 randomised trials, 9,312 adults; adverse events relative risk 1.00; no conflicts declared. PubMed 33234881.
2. Esmaeilinezhad Z, et al. Probiotics for the prevention of Clostridioides difficile-associated diarrhea in adults and children. Cochrane Database of Systematic Reviews, 2025. Cochrane review, 47 randomised trials, 15,260 people; adverse events from 37 trials, 11,911 people; no funding for the update. PubMed 40931979.
3. NHS. Probiotics. Page last reviewed 24 November 2022 (review overdue). nhs.uk/conditions/probiotics.
4. Rao SSC, et al. Brain fogginess, gas and bloating: a link between SIBO, probiotics and metabolic acidosis. Clinical and Translational Gastroenterology, 2018. Observational case series, 30 patients with brain fogginess and 8 without. PubMed 29915215.
5. Quigley EMM, Pot B, Sanders ME. 'Brain Fogginess' and D-Lactic Acidosis: Probiotics Are Not the Cause. Clinical and Translational Gastroenterology, 2018. Letter; authors declare industry interests. PubMed 30250026.
6. 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 randomised trials, 1,182 adults; high heterogeneity and risk of bias noted. PubMed 25099542.
7. Hao Q, et al. Probiotics and acute upper respiratory tract infections. Cochrane Database of Systematic Reviews, 2015. Cochrane review, 12 randomised trials pooled, 3,720 people; odds ratio 0.53 for at least one episode, low-quality evidence. PubMed 25927096.
8. Liu RT, Walsh RFL, Sheehan AE. Prebiotics and probiotics for depression and anxiety: A systematic review and meta-analysis of controlled clinical trials. Neuroscience and Biobehavioral Reviews, 2019. Pooled analysis of 34 controlled trials. PubMed 31004628.
9. Sullivan A, et al. Effect of supplement with lactic-acid producing bacteria on fatigue and physical activity in patients with chronic fatigue syndrome. Nutrition Journal, 2009. Trial, 15 patients with chronic fatigue syndrome, 4 weeks; funding not stated in the abstract. PubMed 19171024.
10. Rao AV, et al. A randomized, double-blind, placebo-controlled pilot study of a probiotic in emotional symptoms of chronic fatigue syndrome. Gut Pathogens, 2009. Randomised pilot trial, 39 patients with chronic fatigue syndrome, 2 months; funding not stated in the abstract. PubMed 19338686.
11. Zhang J, et al. The efficacy of probiotics, prebiotics, and synbiotics on anxiety, depression, and sleep: a systematic review and meta-analysis of randomized controlled trials. BMC Psychiatry, 2025. Pooled analysis; sleep outcome from 12 randomised trials, 789 people; no competing interests declared. PubMed 41310510.

General information, not medical advice. Live cultures are a food supplement; tiredness that lasts is a matter for a GP.