NMN Side Effects: What Human Trials Report

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NMN side effects in published human trials have been few and mostly mild, but those trials were small and short. A 2026 systematic review in Nutrients pooled 10 randomized trials on safety and found no increase in overall adverse events, serious adverse events or liver enzyme changes, at doses of 250 to 2,000 mg a day for 14 days to 24 weeks. Below is what each human trial reported, who was studied, and what the evidence cannot tell you yet.

Nicotinamide mononucleotide (NMN) is a precursor the body uses to make NAD+. My NAD+ vs NMN comparison covers what each one is. I am not a medical professional, and this page is not medical advice.

Table of Contents

What are the side effects of NMN in human trials?

The adverse events that showed up were mostly digestive complaints, colds, skin issues and aches, and they appeared in placebo groups too. A 2026 systematic review found no increase in serious adverse events, and none of the individual papers below reports one. That does not make oral NMN risk free. It means short courses in the groups studied produced no clear safety signal.

What did each NMN trial report?

Most of these studies also measured blood NAD levels. The Okabe, Yi, Igarashi and MIB-626 papers report that NAD levels rose on NMN. A higher blood level is not a proven health benefit, and the safety findings below do not depend on it.

NMN at 250 mg a day for 12 weeks in 30 healthy adults

In a randomized trial in Frontiers in Nutrition, 30 healthy adults (mean age about 43) took 250 mg of NMN daily or placebo for 12 weeks (15 per group). Any adverse event was reported by 8 of 15 on NMN and 7 of 15 on placebo. Gastrointestinal symptoms came up in 3 of 15 on NMN and 2 of 15 on placebo. One person on NMN had abdominal pain that resolved within 30 minutes, and one had hives. There were no serious adverse events, and nobody in the NMN group stopped.

Fever, joint pain or fatigue was reported by 6 of 15 on NMN versus 2 of 15 on placebo. All of these reports started within a day of a COVID-19 vaccination, and the authors suspected the vaccine. Among people who were vaccinated, 6 of 7 on NMN and 2 of 6 on placebo reported it, which was not statistically significant (p = 0.10). With groups this small, treat it as a flag, not a conclusion.

NMN at 1,250 mg a day for 4 weeks in 31 healthy adults

A Scientific Reports safety trial gave 1,250 mg of NMN once daily for up to 4 weeks to healthy adults aged 20 to 65 (16 on NMN, 15 on placebo). Five adverse events were recorded. Loose stool happened once in each group. The NMN group also had a common cold, mild high blood pressure and mild acne. The investigator judged all five unrelated to NMN, and there were no dropouts.

NMN at 300, 600 or 900 mg a day for 60 days in 80 adults

In a GeroScience trial of 80 healthy adults aged 40 to 65, people took 300, 600 or 900 mg of NMN or placebo for 60 days (20 per group). Nine adverse events occurred in seven participants. Placebo had six, the 300 mg group had three (hyperacidity, a skin problem and a mouth ulcer), and the 600 and 900 mg groups had none. The authors reported that none were related to NMN.

One lab result is worth knowing about. LDL cholesterol in the 900 mg group rose from 132.8 to 146.0 mg/dL and was higher than placebo at day 60 (p = 0.02), but that group also started higher than placebo (132.8 versus 114 mg/dL), the paper did not discuss it, and it reported no abnormal lab results. Supplement manufacturers funded this trial and some authors work for them, so read it with that in mind.

NMN at 250 mg a day for 12 weeks in older men

A trial in npj Aging randomized 42 healthy men aged 65 or older to 250 mg of NMN or placebo (21 each). The authors wrote that NMN was well tolerated and that no serious adverse event occurred. A supplier error at week 6 meant 22 participants were left out of the 12-week data, so that analysis rests on 10 people per group.

NMN at 250 mg a day for about 10 weeks in postmenopausal women with prediabetes

The Science trial in postmenopausal women is the one closest to my readers. Twenty-five women with overweight or obesity and prediabetes, with a mean age of 61 to 62, took 250 mg of NMN (13 women) or placebo (12 women) for about 10 weeks. The paper states that no adverse events were reported and that standard blood tests showed no abnormalities in either group.

MIB-626, a form of NMN, at 1,000 or 2,000 mg a day for 14 days

A Journals of Gerontology trial tested a pharmaceutical form of NMN called MIB-626 in 32 adults aged 55 to 80 with overweight or obesity. They took 1,000 mg once daily, 1,000 mg twice daily or placebo for 14 days. The authors reported that it was well tolerated and that adverse event frequency was similar across groups.

Can NMN cause fatigue, nausea or other digestive effects?

Possibly, but the human data do not separate NMN from placebo. Gastrointestinal complaints appeared in 3 of 15 on NMN and 2 of 15 on placebo in the 250 mg trial, and loose stool showed up once in each group in the 1,250 mg trial.

Fatigue is the one to read carefully. In the one trial above that lists fatigue as an event category, it sits in a cluster with fever and joint pain that the authors suspected was a vaccine reaction. I would not call fatigue a proven NMN effect or rule it out. If you start any new supplement and feel worse, stop and call a clinician.

Does NMN cause a niacin flush?

The NIH Office of Dietary Supplements says that nicotinic acid in supplemental amounts can cause skin flushing, and that nicotinamide does not. NMN is a different compound from either one. The adverse-event lists in the 250 mg, 1,250 mg and 60-day trials name no flushing, though small studies can miss uncommon reactions.

Is NMN safe long term?

Nobody can say yet. The 2026 review covered trials lasting 14 days to 24 weeks and concluded that larger and longer trials are needed to confirm long-term safety. The longest single trial described here ran 12 weeks.

The trials also enrolled specific groups: healthy adults, older men, postmenopausal women with prediabetes, and adults 55 to 80 with overweight or obesity. They are not a basis for conclusions about pregnancy, breastfeeding, children or people with serious illness. For what researchers used, see my page on NMN dosage in published trials, which lists doses from studies and not what to take.

Is NMN legal to sell as a dietary supplement?

The FDA’s position changed in 2025. In a September 29, 2025 response to a citizen petition, the agency wrote, “we now conclude that NMN is not excluded from the dietary supplement definition under section 201(ff)(3)(B).” That FDA letter is posted on regulations.gov (read October 19, 2026). A December 2, 2025 FDA letter to an ingredient supplier says the same and sets aside the agency’s November 4, 2022 letter, which had taken the opposite position (read October 19, 2026). Older articles, including an earlier version of this page, describe NMN as excluded from the supplement definition. That is out of date.

What does FDA supplement oversight mean for safety?

Being legal to sell is not the same as being evaluated for safety. FDA’s dietary supplement page (current as of October 1, 2024, read October 19, 2026) says that firms are responsible for evaluating the safety and labeling of their products before marketing, and that the agency can take action against adulterated or misbranded products once they are sold. The September 2025 letter adds that FDA may seek to use seizure and recall tools against any unsafe or adulterated dietary supplement. For you, that means the label and the seller matter, and a bad reaction is worth reporting.

Who should talk to a clinician before taking NMN?

Anyone who is pregnant or breastfeeding, has a current or past cancer diagnosis, has liver or kidney disease, or takes prescription medication should ask a licensed clinician or pharmacist first. I list these groups because the trials above were not built to answer safety questions for them, not because a trial showed harm.

Get medical advice promptly for anything that is not mild, anything that keeps getting worse, or any reaction that looks allergic, such as rash, swelling or trouble breathing. If you are weighing NMN against injections or IV options, my page on NAD+ side effects covers that side of the topic.

A note on medical advice: I research this topic as a woman over 40 and I am not a doctor or a registered dietitian. This page describes published human studies. Your history, conditions and medications decide what is right for you, and only a licensed clinician who knows them can say.

Frequently Asked Questions

Does NMN have side effects?

Published human trials report few, and most were mild: loose stool, stomach discomfort, a cold, a skin problem, a mouth ulcer. They appeared in placebo groups as well, and the trials were short, so no signal is not proof of long-term safety.

Can NMN cause weight gain or weight loss?

The 2026 systematic review found no significant effect of oral NMN on body weight or BMI across the trials that measured them. So there is no trial basis for expecting NMN to move your weight in either direction.

Is NMN safe for women over 40?

Some trials included women, including a 10-week study of 25 postmenopausal women with prediabetes that reported no adverse events at 250 mg a day. That is one small trial in one specific group, so it does not settle the question for every woman. Your clinician can weigh it against your health history.

Can you take NMN with prescription medication?

The trials on this page were designed to measure NAD levels, safety or metabolic effects in the study groups, not drug interactions. Ask your pharmacist or prescriber before adding NMN to any medication you take.

Does NMN make you tired?

One 12-week trial recorded fever, joint pain or fatigue more often on NMN (6 of 15) than on placebo (2 of 15), but the authors linked it to vaccination and the gap was not statistically significant. The evidence is too thin to call fatigue an NMN effect.

Sources

NMN Side Effects: What Human Trials Actually Reported