NMN side effects: what the human safety trials found

In the human trials published so far, NMN has not caused more side effects than a placebo. A 2026 review of 10 controlled trials, at 250 to 2,000mg a day for up to 24 weeks, found no rise in adverse events, serious adverse events or liver enzymes. The honest limit is time: no trial has followed people for longer than about six months.

What the human safety trials of NMN found

Study	Who and how much	How long	What it found on safety
Fukamizu and colleagues, Scientific Reports, 2022	31 healthy adults aged 20 to 65, 1,250mg a day	Up to 4 weeks	No changes beyond normal variation in blood, urine and body measurements. No severe adverse events.
Yi and colleagues, GeroScience, 2023	80 healthy adults aged 40 to 65, 300, 600 or 900mg a day	60 days	No safety issues in adverse event monitoring or lab tests. All 80 people completed the trial.
Yang and colleagues, Nutrients, 2026 (review)	10 controlled trials in adults, 250 to 2,000mg a day	14 days to 24 weeks	No increase in overall or serious adverse events, withdrawals, or liver enzymes (ALT and AST) compared with control groups.
EFSA scientific opinion, 2026	Safety assessment for adults, up to 300mg a day	Review of all evidence	Safe for adults at up to 300mg a day under the proposed conditions, excluding pregnant and breastfeeding women.

I have only listed studies I have read myself, and only what they say about safety. Several NMN trials involve companies that make NMN. Some of Fukamizu's co-authors, for example, work for Mitsubishi Corporation Life Sciences, which produces NMN. That does not make the results wrong, but you should know it.

What the 2026 EFSA opinion means for UK buyers

EFSA, the European Food Safety Authority, published its opinion on β-NMN as a novel food in 2026. It concluded that NMN is safe for adults at up to 300mg a day, excluding pregnant and breastfeeding women. It found no concern for genotoxicity, which is damage to genetic material.

Two things are easy to misread. First, EFSA is the EU's assessor. In Great Britain, NMN is a novel food with an application being assessed by the Food Standards Agency, and EFSA's opinion does not decide that. Second, 300mg was the amount EFSA was asked to assess. It is not a finding that higher amounts are unsafe; the trials in the table above used up to 2,000mg a day without more side effects than placebo. Our Pure NMN is 500mg a capsule, and we tell you that plainly on the label and the product page.

The EFSA panel also noted that in a 90-day study in rats, very high doses affected reproductive organs, and set the no-effect level at 400mg per kilogram of body weight a day. For a 70kg adult that would be 28,000mg a day. The panel judged the safety margin at normal supplement doses sufficient.

Does NMN cause weight gain?

Not in the controlled trials. The 2026 review found no significant change in body weight or BMI in people taking NMN compared with control groups. If you notice your weight changing after starting any supplement, look at what else changed at the same time, and speak to your GP if it continues.

Does NMN cause cancer?

No human trial has reported cancer as a side effect of NMN. But the trials ran for weeks or months, not years, so they could not have picked up a long-term risk either way.

My view is simple. If you have cancer, or have had it, speak to your oncologist before taking NMN or any supplement. That is not something a supplement shop should decide for you.

Who should not take NMN

We do not know your situation, so in these cases our advice is not to take NMN, or to speak to your doctor first.

• Pregnant or breastfeeding women: do not take it, and speak to your GP or midwife. No trial has studied NMN in pregnancy or breastfeeding, and EFSA's 2026 safety conclusion did not cover them.
• Children. Every trial and the EFSA opinion cover adults only, and our NMN is made for adults.
• Anyone taking prescription medicine, who should speak to their GP or pharmacist. Our guide to NMN and medication covers the common ones medicine by medicine.
• Anyone with cancer or a history of it, who should speak to their specialist.

If you notice something after starting NMN

Stop taking it and see whether the problem goes away. If it does, that tells you something. If you want to try again, restart at the dose on the label and change nothing else at the same time, so you know what you are reacting to. If a symptom is severe or does not settle, speak to your GP or call NHS 111.

It also helps to know exactly what is in the capsule. Our Pure NMN is 500mg of NMN in a plant-based capsule with nothing else added, and you can read the purity, heavy metal and microbiology results on our lab results page. A cheap NMN with fillers, or one that has not been tested, gives you more to rule out.