NAD+ precursors

Nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) — precursors that raise cellular NAD+, a coenzyme that declines with age. Heavily marketed off the back of Sinclair-lab mouse work; NR is sold as a supplement, NMN's US status has been contested since the FDA moved to classify it as a drug candidate.

NAD+ is a coenzyme every cell uses to run energy metabolism and to fuel the repair enzymes that fix DNA damage. Tissue levels fall with age, which is the entire premise. You cannot usefully swallow NAD+ itself — it is too large and gets broken down — so the supplements are precursors: NMN and NR, which cells take up and convert back into NAD+. The biology is genuinely sound and the decline with age is real. The unresolved question is the one that matters: raising blood NAD+ is easy and well demonstrated, and no human trial has yet shown that doing so changes an outcome you would care about.

They do raise blood NAD+ — that part is proven. What that DOES for a human is the unanswered part: trials show scattered small wins (some insulin sensitivity in specific groups, some vascular markers) and plenty of nulls. Nothing approaching the mouse rejuvenation story has appeared in people. Ten years of trials have not produced one endpoint worth the price of the powder.

• Multiple randomised trials confirm both NR and NMN reliably raise blood NAD+ levels. That part is settled.
• Translating that into outcomes has repeatedly disappointed, without being literally zero: a 25-woman randomised trial in prediabetes found improved muscle insulin sensitivity (though not systemic), and a 2025 review found limited functional signals in peripheral artery disease. No replicated, clinically meaningful benefit has been established, and the anti-ageing outcome people buy it for has none at all.
• Some small trials report narrow positive signals — walking distance or aerobic measures in specific groups — but they are small, inconsistent and not replicated at scale.
• Short-term tolerability has generally been acceptable — but most published controlled studies ran weeks to a few months, so safety over a year or longer is simply not characterised.
• The US regulatory story on NMN reversed itself: FDA concluded in 2022 that NMN was excluded from the dietary-supplement definition, then in September 2025 reversed that conclusion and determined it is not excluded. That is not an approval — individual products still have to meet supplement requirements — but the "NMN is banned" era is over.
• A theoretical concern that raising NAD+ could support growth of existing cancers is preclinical and unresolved. Effects in people with active cancer are unknown, which is exactly why anyone in treatment should raise it with their oncology team rather than assume either way.

Both NR and NMN are absorbed and raise NAD+ in blood measurably within hours, with tissue effects less well characterised — much of the debate is about whether blood levels reflect what is happening inside muscle, brain and liver. Whether NMN must first convert to NR to cross into cells is still argued. There is no established effective amount because no trial has established an effect worth dosing toward.

Flushing (NR less than niacin), mild GI effects — genuinely low risk. The real costs are money and the one theoretical flag: NAD+ metabolism feeds cancer cells too, so "more is better" is not a settled assumption. Product quality varies wildly; NMN sold after the FDA notice is a regulatory grey zone.

• Anyone with active cancer or in cancer treatment, without oncology input
• Anyone pregnant or breastfeeding — no adequate safety data
• Anyone expecting a felt effect, which no trial supports

• Active cancer or cancer treatment, where effects are unknown and the preclinical growth concern is unresolved — raise it with the oncology team.
• No well-characterised drug interactions are established, which reflects limited study rather than demonstrated safety.
• Niacin and other B3 forms, which feed the same pathway and make stacking largely redundant.

• Nothing you can usefully self-monitor: NAD+ is not on a standard blood panel, and the outcomes people want are not measurable at home.
• Anyone taking it during or after cancer treatment should be doing so with oncology input rather than assuming.
• Whether the product is what it says — independent testing of this category has repeatedly found content that does not match the label.

No withdrawal and no rebound. Blood NAD+ returns toward baseline over days. Since no trial has established a benefit to lose, there is nothing to taper and nothing that has been shown to reverse on stopping.

US: NR is a lawful dietary ingredient; NMN's supplement status is contested (FDA drug-preclusion notice). Both freely sold in practice.

Papers about this compound, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.

• Effects of Supplementation with NAD +  Precursors on Metabolic Syndrome Parameters: A Systematic Review and Meta-Analysis
  Horm Metab Res, 2024
• The Safety and Antiaging Effects of Nicotinamide Mononucleotide in Human Clinical Trials: an Update
  Adv Nutr, 2023
• Nicotinamide riboside and nicotinamide mononucleotide facilitate NAD(+) synthesis via enterohepatic circulation
  Sci Adv, 2025

• The Safety and Antiaging Effects of Nicotinamide Mononucleotide in Human Clinical Trials: an UpdateAdv Nutr · 2023
• Nicotinamide mononucleotide increases muscle insulin sensitivity in prediabetic womenScience · 2021
• NAD+ Precursors Nicotinamide Mononucleotide (NMN) and Nicotinamide Riboside (NR): Potential Dietary Contribution to HealthCurr Nutr Rep · 2023
• Nicotinamide mononucleotide (NMN) as an anti-aging health product - Promises and safety concernsJ Adv Res · 2022
• Long-Term Administration of Nicotinamide Mononucleotide Mitigates Age-Associated Physiological Decline in MiceCell Metab · 2016
• Nicotinamide riboside and nicotinamide mononucleotide facilitate NAD(+) synthesis via enterohepatic circulationSci Adv · 2025
• NAD+ and sirtuins in aging and diseaseTrends Cell Biol · 2014
• Dietary Supplementation With NAD+-Boosting Compounds in Humans: Current Knowledge and Future DirectionsJ Gerontol A Biol Sci Med Sci · 2023
• Regulation of and challenges in targeting NAD(+) metabolismNat Rev Mol Cell Biol · 2024
• NAD(+) Intermediates: The Biology and Therapeutic Potential of NMN and NRCell Metab · 2018
• β-Nicotinamide mononucleotide activates NAD+/SIRT1 pathway and attenuates inflammatory and oxidative responses in the hippocampus regions of septic miceRedox Biol · 2023
• Therapeutic Potential of NAD-Boosting Molecules: The In Vivo EvidenceCell Metab · 2018