Safety and tolerability
Do Probiotics Have Side Effects? What's Normal When You Start
A little extra gas is common. Serious problems are rare, and concentrated in a few groups.
If your first few days on a probiotic come with a bit more gas, you're not doing anything wrong. Here's what the research says is normal, what isn't, and how to start comfortably.
The short answer
For most healthy adults, probiotics are well tolerated, and serious problems are rare.1,2 The most common thing people notice when starting is mild gas or bloating while the gut adjusts, and for most people it fades as the routine settles. Starting gently and staying consistent helps. A few specific groups, including people who are immunocompromised or seriously ill, should talk to a doctor first.3
Probiotics are live microbes, mostly bacteria and sometimes a yeast, that you take to support your gut. They're one of the most popular supplements in the world, and one of the most second-guessed. You take one to feel better, then notice a bit more gas on day three and wonder if something's wrong. Usually, it isn't.
This guide walks through what the research shows about probiotic safety, why that early gas or bloating happens, how to start without the discomfort, and the situations where you should check with a healthcare provider first.
Are probiotics safe?
For most people, the evidence is reassuring. The largest safety review, commissioned by the U.S. Agency for Healthcare Research and Quality, pulled together 622 studies. In the randomized trials, people taking probiotics short-term had no more side effects overall, and no more serious ones, than people on placebo or control. The same review flagged real gaps: side effects were often poorly reported, and long-term effects are largely unknown.1
A second systematic review concluded that the overwhelming evidence suggests probiotics are safe, while noting that the serious problems it did find, such as bloodstream infections, came mostly from critically ill, hospitalized, post-surgery and immune-compromised patients.2
The important nuance: "safe for most people" isn't the same as "safe for everyone in every situation." The serious risks that do exist are concentrated in specific, identifiable groups, which we cover below.3
What's normal when you start
Mild gas, bloating or a change in how often you go during the first week or two is the most commonly reported experience. In a small 10-day study of 10 healthy adults taking a high-dose multi-strain probiotic, everyone finished, there were no serious adverse events, and the symptoms people reported were mostly digestive.4 Here's why it can happen.
Why
A little more gas
Carbohydrates your body doesn't absorb are fermented by bacteria in your colon, which makes short-chain fatty acids and gas.7 More fermentation can briefly mean more gas.
Why
Your gut adjusts
Adding new strains nudges the balance of your gut community. That shift can feel like temporary bloating or a change in stool before things settle.
What to expect
It usually passes
For most people these sensations are mild and short-lived. Want the full picture? See how long probiotics take to work.
| What people report | Why it can happen | What tends to help |
|---|---|---|
| Gas or bloating | Bacterial fermentation makes gas, and more activity in the gut can mean more of it.7 | Start gently, take it consistently, give it a week or two. |
| Changes in stool or regularity | The gut community is adjusting to new strains. | Stay hydrated and keep the routine steady day to day. |
| Mild cramping | A temporary shift in gut activity while you adjust. | Take it with food; step back to a gentler start if needed. |
When to pay attention
Mild and improving is the usual pattern. If symptoms are severe, or they haven't eased after a couple of weeks of consistent use, stop and check with your healthcare provider rather than pushing through.
How to start probiotics without the bloat
You can stack the odds in your favor. The goal is to give your gut an easy on-ramp instead of a sudden jump.
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1
Start gentle
Begin with a moderate-potency probiotic and let your gut meet the strains gradually.
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2
Be consistent
Take it daily, ideally with food, so the routine does the work.
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3
Then build
Step up to a higher-potency daily formula once you feel settled.
A couple of formula details make the on-ramp smoother. A lower-potency entry probiotic is an easy place to begin, and a delayed-release capsule is designed to help more of the cultures survive stomach acid and reach the intestinal tract.* Timing matters too, which we cover in when to take probiotics. If you're adding fiber at the same time, ease it in gradually rather than all at once; see the gentle fiber approach.
An easy place to begin, and where to go next
start
Gut Balance | Organic Flora
30B cultures, 60 strains, USDA Organic
A balanced, entry-level organic probiotic made for adults who are new to probiotics and want broad-spectrum daily support.*
Ultra Daily | Vital Flora
60B cultures, 60 strains, delayed-release
When you're ready to step up: 60 billion cultures in a delayed-release capsule designed to protect them from stomach acid.*
Not sure which fits? The quiz below routes you in three questions.
How should you start probiotics?
Find your on-ramp in three questions
Based on the research in this article. Not a diagnosis, and not a substitute for your provider's advice.
Have you taken a daily probiotic before?
How sensitive is your stomach to gas and bloating?
How's your daily fiber intake?
Your best starting point
Ease in first, then feed your gut
A sensitive stomach, a new probiotic and more fiber all at once is a lot of change. Start with a gentle daily probiotic on its own for a couple of weeks, then add fiber slowly once you feel settled.*
Your best starting point
Ease in with a gentle daily
Starting gently gives a sensitive gut the smoothest on-ramp. A moderate-potency, broad-spectrum daily lets you build a routine before stepping up, which is the simplest way to keep that early adjustment small.*
Your best starting point
Start gentle and feed your gut
Your digestion is steady, so you can start a gentle daily probiotic and bring fiber up alongside it. Go slowly with the fiber, since more fiber means more fermentation. A fiber designed to be easier on gas and bloating makes that transition kinder.*
Your best starting point
Start gentle, step up in a few weeks
Steady digestion and good fiber make for an easy start. Begin with a gentle daily probiotic, and if all feels settled after a few weeks, step up to a higher-potency daily in a delayed-release capsule.*
Your best starting point
Keep it steady before stepping up
You're used to probiotics but your stomach is still sensitive. Keep taking a gentle daily with food and on a steady schedule. If you want to move to a higher-potency formula, switch once things feel calm, not during a bloated stretch, and check our guide to what else helps with bloating.*
Your best starting point
Full-strength daily, plus fiber
Your digestion is steady and you've done this before, so a higher-potency daily makes sense. A delayed-release capsule is designed to protect the cultures from stomach acid so more reach the intestinal tract.* The gap is fiber, so add it gradually.
Your best starting point
Go for a full-strength daily
Your digestion is steady, your fiber is in good shape and you've done this before, so a higher-potency daily makes sense. A delayed-release capsule is designed to protect the cultures from stomach acid so more reach the intestinal tract.*
Who should be cautious
This is the part that matters most. Probiotics are well tolerated by most people, but the rare serious risks are concentrated in specific clinical situations.3 Here's what the research found in each.
| Situation | What the research found |
|---|---|
| Critically ill or in intensive care | In a trial in 298 patients with predicted severe acute pancreatitis, a multispecies probiotic given through the gut didn't reduce infections and was linked to more deaths than placebo (16% vs 6%).8 A later meta-analysis of ICU trials found no difference in deaths and fewer ICU-acquired infections with probiotics,12 so this is a decision for the care team, not a supplement to start on your own. |
| Central venous catheter or tube feeding | A review of patients on nutritional support found 32 cases of infection from L. rhamnosus GG or Saccharomyces boulardii, with central venous catheters among the risk factors.10 |
| Probiotic yeast in hospital | Seven intensive-care patients, two of them premature newborns, developed yeast bloodstream infections that matched the Saccharomyces probiotic they'd been given.11 |
| Premature infants and young children | Across 49 reported cases of invasive infection linked to probiotics in children, most were under 2, and nearly all had a risk factor such as prematurity or an IV catheter.13 |
| Immune-compromised adults | A review of 57 studies in adults with HIV, critical illness, surgery, or organ or autoimmune disease found the strains and doses tested were safe, but called the reporting of side effects inconsistent.9 |
| Inflammatory bowel disease | In a meta-analysis of 9 trials in adults with IBD, abdominal pain was more common on probiotics than on placebo.6 |
Check with a provider first
Talk to a healthcare provider before starting if you're immunocompromised or on immune-suppressing medication, critically ill or hospitalized, have a central venous catheter or recent major surgery, or are caring for a premature infant. If you're pregnant, nursing, or managing a health condition, it's worth a quick conversation too.
For most other adults, the reviews found probiotics largely safe at the strains and doses tested, while recommending medical guidance for vulnerable people.2,9 The takeaway is to match the decision to your situation.
Frequently asked questions
For most healthy adults, probiotics are well tolerated and serious side effects are rare. In short-term randomized trials, side effects were no more common on probiotics than on control.1 The most common experience when starting is mild gas or bloating while the gut adjusts, which fades for most people as the routine settles.
Bacteria in your colon ferment carbohydrates your body didn't absorb, and gas is one of the byproducts.7 Adding new strains can briefly change that activity, which can feel like extra gas or bloating while your gut adjusts.
When mild gas or bloating shows up, it's usually short-lived and eases with consistent daily use. If symptoms are severe or haven't improved after a couple of weeks, stop and check with your healthcare provider.
For generally healthy adults, the research supports daily use in the short term, and consistency is how probiotics are meant to be taken. Long-term effects are less studied.1 If you have a health condition or take medication, confirm with your provider.
People who are immunocompromised, critically ill or hospitalized, have a central venous catheter or recent major surgery, and premature infants should only use probiotics under medical guidance, because rare serious infections have been reported in these groups.2,10,13 When in doubt, ask a provider first.
Some people notice changes in stool or regularity in either direction while their gut adjusts. Staying hydrated and keeping a steady daily routine usually helps things normalize. If it persists, step back to a gentler formula or speak with your provider. For probiotics studied for regularity, see probiotics for constipation.
Many people find taking a probiotic with food is easiest on the stomach, and a delayed-release capsule is designed to help cultures survive stomach acid whenever you take it.* For a full breakdown, see our guide to when to take probiotics.
Start with a lower-potency probiotic, take it consistently with food, give your gut a week or two to adjust, and if you're adding fiber, increase it gradually rather than all at once. A delayed-release capsule can also make the start gentler.*
Both can be true at different stages. In the first days or weeks you may notice a temporary uptick in gas as your gut adjusts. Over longer use, some strains have been studied for the opposite: in a six-week trial in 76 healthy adults, one Bacillus subtilis strain improved a combined score for bloating, burping and flatulence compared with placebo.5 The effect is gradual and depends on the strain. For a deeper look, see our guide to probiotics for gas and bloating.
Saccharomyces boulardii is a probiotic yeast that most people tolerate well, with mild gas or bloating the most common report. Because it's a live yeast, it has caused rare bloodstream infections in hospitalized and intensive-care patients, especially those with central venous catheters.10,11 People who are critically ill, immunocompromised or have a central line should avoid it unless a doctor advises otherwise, and anyone with a yeast allergy should skip it.
Gummies are easy to take, but many add sugars or sugar alcohols like xylitol or erythritol, which can cause gas or bloating on their own in sensitive people. They can also carry fewer cultures than a capsule, so check the label. A delayed-release capsule is designed to protect cultures from stomach acid so more reach the intestinal tract.* If capsules are difficult for you, a low-sugar, third-party-tested gummy is a reasonable alternative.
Many people use probiotics during pregnancy, but pregnancy is exactly the kind of situation where you should confirm with your own healthcare provider before starting any supplement, so the choice fits your circumstances.
References
- Hempel S, et al. Safety of probiotics used to reduce risk and prevent or treat disease. Evid Rep Technol Assess (Full Rep). 2011(200):1-645. PubMed
- Didari T, et al. A systematic review of the safety of probiotics. Expert Opin Drug Saf. 2014;13(2):227-39. PubMed
- Doron S, Snydman DR. Risk and safety of probiotics. Clin Infect Dis. 2015;60 Suppl 2:S129-34. PubMed
- Ryan JJ, Patno NM. Short-Term Tolerability, Safety, and Gut Microbial Composition Responses to a Multi-Strain Probiotic Supplement: An Open-Label Study in Healthy Adults. Integr Med (Encinitas). 2021;20(1):24-34. PubMed
- Garvey SM, et al. The probiotic Bacillus subtilis BS50 decreases gastrointestinal symptoms in healthy adults: a randomized, double-blind, placebo-controlled trial. Gut Microbes. 2022;14(1):2122668. PubMed
- Dore MP, et al. Side Effects Associated with Probiotic Use in Adult Patients with Inflammatory Bowel Disease: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients. 2019;11(12). PubMed
- Fernández-Bañares F. Carbohydrate Maldigestion and Intolerance. Nutrients. 2022;14(9). PubMed
- Besselink MG, et al. Probiotic prophylaxis in predicted severe acute pancreatitis: a randomised, double-blind, placebo-controlled trial. Lancet. 2008;371(9613):651-659. PubMed
- Van den Nieuwboer M, et al. The administration of probiotics and synbiotics in immune compromised adults: is it safe? Benef Microbes. 2015;6(1):3-17. PubMed
- Whelan K, Myers CE. Safety of probiotics in patients receiving nutritional support: a systematic review of case reports, randomized controlled trials, and nonrandomized trials. Am J Clin Nutr. 2010;91(3):687-703. PubMed
- Roy U, et al. Seven cases of Saccharomyces fungaemia related to use of probiotics. Mycoses. 2017;60(6):375-380. PubMed
- Alsuwaylihi AS, McCullough F. The safety and efficacy of probiotic supplementation for critically ill adult patients: a systematic review and meta-analysis. Nutr Rev. 2023;81(3):322-332. PubMed
- D'Agostin M, et al. Invasive Infections Associated with the Use of Probiotics in Children: A Systematic Review. Children (Basel). 2021;8(10). PubMed