NMN side effects: what trials and EFSA report

People search for NMN side effects because the ingredient is talked about as if it were a vitamin. It is not an authorised novel food in Great Britain. This page is about harm and uncertainty in the documents cited here. It does not tell you a safe dose. The Food Standards Agency application RP-2116 was still in risk assessment on the FSA record. Short trials in selected adults are not a leaflet for pregnancy, cancer treatment, or years of daily use. Ask a GP or a pharmacist before you start.

What short human trials reported

In the Yoshino trial, 13 postmenopausal women with prediabetes took 250 mg a day for about 10 weeks and 12 took placebo. The paper says no adverse events were reported and standard blood tests showed no abnormalities in either group. That is a real statement about a small, short study. It is not a certificate for the general population. The trial was not designed as a side-effect study with thousands of people.

Yi and colleagues gave 80 healthy middle-aged adults placebo or 300, 600 or 900 mg a day for 60 days. They wrote that they found no safety issues based on adverse events and on laboratory and clinical measures, and that the supplements were well tolerated. Again, the follow-up is two months. A side effect that needs a year, or that appears in people who were excluded from a healthy-volunteer trial, would not show up.

What the European safety assessment added

EFSA's 2026 opinion is the longest safety document cited here. It covers one production method: chemical synthesis, purity at least 99%, not every powder on the internet. The panel did not identify a genotoxicity concern. In a 90-day rat study it did see effects on reproductive organs. The no-observed-adverse-effect level was 400 mg per kilogram of body weight a day. The proposed human use was 300 mg a day, and the panel thought the margin of exposure was enough for adults excluding pregnant and lactating women. The opinion says the human study supplied did not raise a safety concern. It also says 300 mg of that NMN lines up with about 109.7 mg of nicotinamide, under the 900 mg upper level for nicotinamide and well above a typical niacin reference intake.

Read the exclusions literally. Pregnant women and women who are breastfeeding were not in the proposed use. The rat finding is why a casual 'it's a vitamin' sentence is wrong. EFSA was not asked to decide whether NMN prevents aging, and the opinion does not. A favourable safety opinion is not permission to sell the ingredient in Great Britain. RP-2116 has not finished risk assessment.

Who should not treat blogs as a clearance

• Pregnancy and breastfeeding: excluded from the EFSA proposed use.
• Children: not the population in Yoshino or Yi, and not the EFSA target population.
• People with cancer, or with a history of cancer: NAD chemistry is active in many cells, including cells you may not want to feed. That is a reason to ask an oncologist, not a reason for us to invent a risk number.
• People taking prescribed medicines: neither trial is an interaction study.
• Anyone using an unauthorised product bought online: quality, dose and identity were not part of Yoshino's capsule, which came from a named supplier inside a trial.

The opposite pattern is not implied either. The honest summary is: short trials at 250 to 900 mg looked tolerable in the people who were allowed in, animal reproductive findings exist for the ingredient EFSA assessed, and Great Britain has not authorised the food. Call 999 in an emergency and NHS 111 when you need urgent advice that is not life-threatening. A GP or pharmacist can look at what you are actually taking. Nothing here is a personal dose. Short trials are not a leaflet for pregnancy, cancer treatment, or years of daily use. Ask a clinician first. Call 999 in an emergency and NHS 111 when the problem is urgent and not life-threatening. A GP or pharmacist can look at the other medicines.

United States letters are not a UK licence

The FDA letter of 2 December 2025 says NMN is not excluded from the US dietary-supplement definition, reversing the agency's 2022 view. That letter is American. It does not authorise a novel food in Great Britain, and it does not erase side effects. Nicotinamide riboside chloride remains the related ingredient that does have a GB novel-food authorisation, with maximums of 300 mg or 230 mg a day depending on pregnancy. That authorisation is a specific fact about nicotinamide riboside chloride. 'Not authorised' is a specific fact about NMN, not a mood.

Stop and get help if you feel seriously unwell after any supplement. Call 999 in an emergency and NHS 111 for urgent advice that is not life-threatening. A GP or pharmacist can look at what you are actually taking. Readers sometimes want a single list of side effects the way a medicine leaflet has one. NMN does not have that leaflet in the UK, because it is not an authorised medicine and it is not an authorised food. The closest documents are the two short trials and the EFSA opinion on one ingredient. Mild stomach upset, headache or a flush are often repeated on forums. The record is the absence of reported events inside two small trials, the rat reproductive finding, the exclusion of pregnancy and breastfeeding, and the nicotinamide arithmetic. If a seller's page adds a longer side-effect list, ask which trial it came from. If the answer is a blog, it is not a trial. The nicotinamide arithmetic is worth one more pass. EFSA said 300 mg of the NMN it assessed corresponds to about 109.7 mg of nicotinamide. The upper level it cites for nicotinamide in adults is 900 mg a day. A single day at that proposed maximum is not the upper level. It is still several times a typical niacin reference intake, on top of diet and any multivitamin. Stacking was not the design of the human trials. If you already take niacin, a B complex, or nicotinamide riboside chloride, which is the related ingredient Great Britain has authorised, adding an unauthorised NMN product is outside what these trials describe. The evidence page next to this one sets out the legal status. Read them together. If you feel unwell, use 999 or NHS 111 rather than a search result. A pharmacist can see the actual box. These trials do not list the other ingredients in a multivitamin, and they are not a reason to stop a prescribed medicine.