Urolithin A Side Effects: What Trials Show

The short answer

Urolithin A has a reassuringly clean safety profile in the human research so far. The first-in-human trial was built specifically to look for harm and found none of note, at single doses up to 2,000 mg and at up to 1,000 mg a day for four weeks. Two later four-month randomized trials in older and middle-aged adults reported side effects at rates similar to placebo. When side effects do show up they are mild and digestive. It is also a compound your own gut bacteria make from pomegranates, walnuts and berries, which is part of why the body treats it as familiar.

The honest caveats are about what has not been studied rather than anything that has gone wrong: no trial has run longer than about four months, no formal drug-interaction studies exist, and there is no data in pregnancy. For what urolithin A does and the evidence behind it, see the full urolithin A guide.

What the trials found on safety

Study	Who and how much	Duration	Safety finding
Andreux 2019, Nature Metabolism	Healthy older adults; single doses 250 to 2,000 mg, then 250, 500 or 1,000 mg/day	Single dose, then 28 days	Safe and well tolerated at every dose; adverse events similar to placebo
Liu 2022, JAMA Network Open	66 adults aged 65 to 90; 1,000 mg/day	4 months	No safety concerns; adverse events similar between groups
Singh 2022, Cell Reports Medicine	88 middle-aged adults; 500 or 1,000 mg/day	4 months	Well tolerated; no treatment-related serious adverse events
Heilman 2017, Food and Chemical Toxicology	Animal and lab toxicology package	Up to 90 days in rats	Not genotoxic; no adverse effects at the highest dose tested, several grams per kg of body weight per day

That last row is what supported the US regulatory position: urolithin A went through the FDA’s GRAS notification process, and the agency responded with no questions in 2018. GRAS is a food-safety standard for the intended use, not a judgment that it works, but it does mean the toxicology was reviewed rather than assumed.

⚠ CLAIM (wellness): In human trials of up to four months at 250 to 1,000 mg/day, urolithin A was well tolerated with adverse events similar to placebo. Basis: Andreux 2019; Liu 2022; Singh 2022 (Cell Rep Med). Long-term (multi-year) safety has not been studied. Lane: educational.

Possible side effects

When side effects do show up, they are usually mild and digestive, the usual suspects for any oral supplement:

• Mild digestive upset: occasional bloating, gas, loose stools or stomach discomfort.
• Nausea: uncommon, and usually mild when it happens.

The key phrase from the trials is “similar to placebo.” In a controlled study, people taking the placebo also report bloating and headaches, because people get bloating and stomach upsets for all sorts of reasons. When the two groups report them at similar rates, the side effects cannot be clearly pinned on urolithin A.

Taking it with food can help minimize any digestive discomfort.

Doses used in the studies

The trials behind this safety profile mostly tested 500 mg to 1,000 mg per day, taken as a single daily dose, with single doses up to 2,000 mg in the first-in-human study. Side effects did not scale up meaningfully at the higher end of that range, which is part of why the data reads as reassuring rather than dose-dependent. The 250 mg dose was also safe but did not move the mitochondrial gene signature in the first trial, which is why commercial products cluster at 500 mg and above. Starting at the lower end before moving up is a reasonable approach if you tend to be sensitive to new supplements generally.

Who should be cautious

Even with a clean profile, a few groups should get medical guidance first:

• Pregnant or breastfeeding: there is no human data at all, so the default is to avoid.
• People on medications: no formal drug-interaction studies have been published. Urolithin A is cleared mainly by being bound to glucuronide and sulfate groups in the liver and gut, a common route for drugs too, so an interaction is theoretically possible even though none has been reported.
• People with hormone-sensitive conditions: in lab studies, urolithins showed weak estrogen-like and anti-estrogen activity in cultured cells. Nothing like this has been seen in humans, and the concentrations involved may not be reached in the body, but it is a reasonable reason to ask a clinician first if you have a history of hormone-sensitive cancer.
• Anyone with a current cancer diagnosis: urolithin A affects cell recycling (mitophagy) and immune cells, and its effects in people with cancer are not established. This is a conversation for your oncologist, not a label.

⚠ CLAIM (wellness): Urolithins showed weak estrogenic and anti-estrogenic activity in cell culture; this has not been observed in humans and its clinical relevance is unknown. Basis: Larrosa 2006, J Agric Food Chem. Lane: educational, flagged for clinician discussion.

Who should be cautious, at a glance

Group	Guidance
Pregnant or breastfeeding	Avoid; no safety data for this group
On medication or managing a health condition	Check for interactions with a clinician before starting; none has been formally studied
History of hormone-sensitive cancer, or a current cancer diagnosis	Ask your oncologist first; lab signals only, but effects in this group are unknown
New to supplements generally	Start at 500 mg rather than the higher end of the studied range
Everyone else	Generally well tolerated at studied doses, per the trial data above

Long-term safety

Human trials have run for up to about four months without notable safety signals, which is encouraging. As with most newer supplements, multi-year data does not yet exist. That is a reasonable caveat, not a red flag. It helps that people who eat a lot of pomegranate, walnuts and berries, and who have the right gut bacteria, have been producing urolithin A in their own bodies for as long as people have eaten those foods, though typically at lower levels than a 500 mg supplement delivers.

Food versus supplement: a safety difference?

Only some people can make meaningful amounts of urolithin A from food. Gut bacteria convert ellagitannins from pomegranates and nuts into urolithin A, and studies of these “metabotypes” find a substantial share of people produce little or none. Supplements bypass that, which is the main argument for them. From a safety standpoint the molecule is the same either way; the difference is the dose, which is why trial data at supplement doses, not dietary history, is the evidence that matters. More on food sources in urolithin A foods.

Choosing a product safely

Most safety problems with supplements come from the product, not the molecule. The trial data applies to urolithin A at the stated dose and purity, so it only covers you if the bottle contains that. Look for third-party testing: a batch-specific certificate of analysis from an independent lab confirming the urolithin A content and purity, and screening for heavy metals and residual solvents. Urolithin A is synthesized rather than extracted, so purity and residual solvents from manufacturing are the things worth checking. Our buyer’s guide is at best urolithin A supplement.

What people report

Community reports, kept separate from the trial evidence above. These are not evidence that urolithin A causes or prevents any of these things.

• The most common report is no side effects at all, which lines up with the trials.
• A minority describe mild bloating or looser stools in the first week or two, which they say settles. Taking it with a meal is the usual fix.
• Occasional reports of headaches or vivid dreams appear in forums. Neither was a signal in the trials.
• Some users report better exercise endurance or less fatigue after a few months. The trials found mixed results here, with primary endpoints missed and some secondary measures improved.
• Price is the most common reason people stop, not side effects.

Bottom line

Urolithin A is among the better-tolerated supplements in the mitochondrial space, with side effects that are mild, uncommon, and often indistinguishable from placebo in trials up to four months at 500 to 1,000 mg a day. The gaps are long-term data, drug interactions and pregnancy, none of which have been studied. Take it with food, stick to studied doses, buy a third-party tested product, and if you are pregnant, breastfeeding, on medication, or have a hormone-sensitive or current cancer, clear it with a clinician first.

Educational information only, not medical advice, and not evaluated by the FDA. Urolithin A is not a treatment for any disease. Consult a clinician if you are pregnant, breastfeeding, taking medication, or managing a health condition.