Here's the short version. The selected trials of NMN generally report good short-term tolerability. They are too small and short to rule out uncommon or delayed harms, or establish safety in excluded groups.
What the human trials found
Across trials of a single dose up to 24 weeks, NMN was well tolerated, with no treatment-related serious side effects reported. The table summarizes selected trials.
| Trial | People | Length | Amount the trial used | Safety result | Sponsor or industry ties |
|---|---|---|---|---|---|
| Irie 2020 [2] | 10 healthy men (no placebo group) | Single dose | 100, 250 and 500 mg, one at a time | No significant symptoms; small lab shifts stayed in normal ranges | Co-authors from Shionogi |
| Fukamizu 2022 [1] | 31 healthy adults, 20 to 65 | 4 weeks | 1,250 mg a day | No changes beyond normal variation; no severe side effects | Funded by Mitsubishi Corporation Life Sciences |
| Okabe 2022 [3] | 30 healthy adults | 12 weeks | 250 mg a day | No abnormal lab or exam results | NMN supplied by Mitsubishi Corporation Life Sciences |
| Igarashi 2022 [4] | Healthy older men | 6 or 12 weeks | 250 mg a day | Well tolerated, no significant harmful effect | Mitsubishi Corporation Life Sciences co-authors |
| Yi 2023 [5] | 80 healthy middle-aged adults | 60 days | 300, 600 or 900 mg a day | No safety issues on adverse-event, lab and clinical checks | First author works for Abinopharm |
| Pencina 2023 [6] | 32 adults aged 55 to 80 with overweight or obesity | 14 days | 1,000 mg of MIB-626 once or twice a day | Side effects as frequent as on placebo | Metro International Biotech co-authors |
| Katayoshi 2023 [9] | 36 healthy middle-aged adults | 12 weeks | 250 mg a day | Well tolerated, no side effects reported | Authors report no competing interests |
| Morifuji 2024 [7] | 60 older adults | 12 weeks | 250 mg a day | No side effects linked to NMN | Authors work for Meiji |
| Akasaka 2023 [8] | 14 men over 65 with diabetes | 24 weeks | 250 mg a day | Tolerated, no severe side effects | Not stated in the abstract |
Amounts are what each trial gave, listed so you can see what was tested. They aren't a recommended dose. All trials are placebo-controlled unless the row says otherwise.
That's a consistent picture. It's also a narrow one: a few hundred people in total, mostly healthy, and mostly in Japan.
Is NMN safe to take daily?
The cited trials include daily use for up to 24 weeks without a clear safety signal. That does not establish safety over years or exclude uncommon adverse effects.
One dedicated safety trial used 1,250 mg daily for 4 weeks [1]. The 24-week trial in this table enrolled only 14 men [8]. Pencina also tested 1,000 mg twice daily for 14 days [6]. These separate, small studies do not establish safety at high amounts for long periods.
What short, small trials can and can't catch
They can catch common, early problems; they can't catch rare or slow ones. If a side effect showed up in one person in a hundred, a 30-person trial could easily miss it.
Problems that build over years, like effects on blood vessels, organs or cancer risk, need large trials or long tracking of real-world users. The selected NMN trials do not provide that evidence. Supplements are not subject to the same premarket drug-approval process, and small trials do not resolve rare-event or long-duration risks.
Side effects people report
Trials didn't find more side effects on NMN than on placebo. In the single-dose study, heart rate, blood pressure, oxygen levels and body temperature didn't change [2].
You'll still see people describe side effects online. Individual reports cannot, by themselves, distinguish an NMN effect from another ingredient, another treatment or coincidence. If something feels off after you start, stop and tell your clinician.
Who should be careful, or skip it
The selected studies do not establish safety for pregnancy, breastfeeding, cancer treatment, or combinations with other medicines. Ask a clinician first.
- Pregnancy and breastfeeding: the studies summarized here do not establish safety.
- Cancer: none of the trials above reported a cancer signal, but they were far too small and short to detect one. If you have cancer or are being treated for it, that's a conversation with your oncologist.
- Long-term conditions: the table includes one small trial in older men with diabetes [8]. That does not establish safety across kidney, liver or heart conditions.
- Teens and children: these adult trials do not establish pediatric safety.
Can you take NMN with other supplements or medicines?
The selected studies do not establish a drug-interaction profile. That's not the same as "no interactions."
The linked PubMed search can help locate interaction research [11], but the studies summarized here do not establish NMN's interaction profile alongside common medicines. Tell your clinician and pharmacist everything you take, including other supplements.
If you're thinking of combining it with a NAD+ product, see NMN and NAD+ together.
Does NMN cause weight gain?
The selected trials did not identify a clear weight-gain effect; they cannot rule out an individual change. In the 4-week safety trial, body measurements stayed within normal variation [1]. In older men over 6 to 12 weeks, body composition didn't change [4].
What animal studies add, and don't
Rat studies found no harmful effects at moderate amounts, but very high amounts reduced body weight; rat results don't set a safe human dose.
In a 91-day rat study of one commercial NMN, the highest level without harmful effects was 500 mg per kilogram a day in males and 1,000 in females. At 2,000, rats ate less and gained less weight. The study was written by staff of a testing lab and a US company [10]. Those numbers describe rats; they can't be turned into a human dose.
Product quality is a safety issue too
Because FDA doesn't test supplements before sale, what's in the bottle depends on the maker. The trials used products made for research, often supplied by the company that funded them.
NMN's legal status has also moved. In September 2025, FDA concluded that NMN isn't excluded from the definition of a dietary supplement, reversing its 2022 position, in its primary petition response [12] [13]. Each maker is still responsible for safety and for an accurate label.
Before you buy, check our guide to where to buy NMN and the questions in is NMN legit.
The bottom line
The small trials generally found good short-term tolerability. That does not establish that an individual product or long-term regimen is low-risk for you. If you decide to try it, tell your clinician, keep a list of everything you take, and stop if anything changes that worries you. Whether it's worth taking at all is a separate question, covered in does NMN work and how long NMN takes to work. If you've already stopped, why people stop taking NMN may sound familiar.