Across 366 first-hand reports, people mentioned a side effect 16 times (about 4 per 100 reports). The most common: insomnia, jitteriness, drowsiness. User reports show what people notice — they can't tell you whether NMN is safe for you.
“I started taking NMN a few months ago and felt the same better sleep more steady energy and clearer thinking.”
“I've been supplementing with NMN and trans-resveratrol for awhile now and I take 1g of each a day. I noticed I have insane amounts of energy.”
“I experimented with NMN 3 years ago and had sleeplessness as a side effect after a few days and never tried it again. (I did increase my mile run time by 45 seconds in the first week though)”
Conclusion: Potential risks in raising NAD+ levels in various clinical disorders using NAD+ precursors include the accumulation of putative toxic metabolites, tumorigenesis and promotion of cellular senescence. Therefore, NAD+ metabolism represents a promising target and further studies are needed to recapitulate the preclinical benefits in human clinical trials.