NAD precursors: what NMN and NR trials actually show

Key takeaways

• NMN and NR are NAD+ precursors. Both reliably raise blood NAD+ surrogate markers in human pharmacokinetic trials at typical supplement doses (250-1000 mg daily) .
• The FDA reversed its 2022 exclusion of NMN in September 2025 and now treats NMN as not excluded from the dietary-supplement definition; NMN stays a New Dietary Ingredient that requires premarket notification .
• Nicotinamide riboside chloride (Niagen / Tru Niagen) has the longer regulatory record, with FDA GRAS no-questions status since 2016 and acknowledged NDI notifications .
• Almost all published NMN and NR trials use surrogate endpoints (NAD+ levels, insulin sensitivity, blood pressure); a 2024 meta-analysis of 12 NMN trials found most clinically relevant outcomes were no different from control .
• Adverse-event profiles at studied doses are mild across both compounds; long-term safety beyond about 12 months of continuous dosing is not characterized .

The NAD biology, briefly

Nicotinamide adenine dinucleotide (NAD+) is a redox cofactor central to oxidative phosphorylation, and it is also the substrate for NAD-consuming enzymes including the sirtuins, the PARP family, and the CD38 cell-surface ectoenzyme. Tissue NAD levels decline modestly with age in some animal and human cross-sectional data; CD38 expression rises with age, which is one mechanistic explanation for the decline. That combination drove the modern interest in NAD precursors as a way to support tissue NAD levels .

Five precursors feed into the NAD salvage and de novo pathways: tryptophan (de novo), nicotinic acid (NA / niacin), nicotinamide (NAM), nicotinamide riboside (NR), and nicotinamide mononucleotide (NMN). NAM and NA have decades of human clinical use at gram doses (pellagra prevention, lipid management). NR and NMN are the newer precursors that anchor the modern longevity-supplement conversation.

NMN vs NR: the biosynthetic difference

Once inside the cell, both NR and NMN funnel into the NAD salvage pathway. The pathways diverge at the cell membrane. Oral NR is taken up via nucleoside transporters and converted to NMN by nicotinamide riboside kinase (NRK1/NRK2), and NMN is then converted to NAD+ by NMN adenylyltransferase (NMNAT). Oral NMN is debated: NMN can be dephosphorylated to NR at the brush border, or transported intact via the SLC12A8 transporter that one influential preclinical study identified, though that transporter's role is contested in the published literature .

So oral NMN may largely act through the same intracellular salvage as oral NR. Whether NMN offers any pharmacokinetic or pharmacodynamic advantage over NR in humans is not settled in the published trial record.

NR (nicotinamide riboside): the human trial record

Trammell and colleagues (2016) published the first oral NR pharmacokinetic trial in 12 healthy adults, showing dose-dependent increases in NAD+ measured in peripheral blood mononuclear cells (PBMCs) after 100 mg, 300 mg, and 1000 mg single doses . Conze and colleagues (2019) extended this with an 8-week safety and tolerability study at up to 1000 mg/day, reporting dose-dependent increases in whole-blood NAD+ (22, 51, and 142 percent at the 100, 300, and 1000 mg doses) that were maintained throughout dosing, with a clean adverse-event profile .

Martens and colleagues (2018) ran a 6-week randomized crossover in healthy middle-aged and older adults at 1000 mg/day; PBMC NAD+ rose about 60 percent. Whole-group systolic blood pressure and aortic stiffness showed only non-significant downward trends; a post-hoc subgroup with elevated baseline blood pressure had roughly 9 mmHg lower systolic pressure, an exploratory signal the trial was not powered to confirm .

The NADPARK trial extended NR to Parkinson's disease and reported that NR raised cerebral NAD levels measured by 31-phosphorus magnetic resonance spectroscopy, an effect associated with mild clinical improvement in the responders . Other NR trials have looked at mitochondrial function in older adults and tolerability in chronic kidney disease, with the same pattern throughout: NR reliably raises blood and tissue NAD+ surrogate markers, and the effects on functional or outcome endpoints are smaller and more variable.

NMN: the human trial record

Irie and colleagues (2020) published the first single-dose NMN PK study in 10 healthy Japanese men, showing tolerability up to 500 mg and increased downstream NAD-metabolite markers . Yoshino and colleagues (2021) ran a 10-week randomized trial of NMN 250 mg/day in 25 postmenopausal women with prediabetes; skeletal-muscle insulin sensitivity (measured by hyperinsulinemic-euglycemic clamp) improved in the NMN arm, without large changes in body composition or blood pressure .

Liao and colleagues (2021) ran a 6-week randomized trial of NMN at 300, 600, and 1200 mg/day in recreational runners; the 600 and 1200 mg groups improved several aerobic-capacity measures (oxygen uptake and power at the ventilatory thresholds) over placebo, though absolute VO2max did not differ between groups . A separate dose-finding study of an oral NMN formulation (MIB-626) in adults aged 55 to 80 reported dose-related increases in circulating NAD+ over two weeks .

All of these trials are surrogate-endpoint, short-duration, and small. None establishes NMN as a longevity or healthspan therapy.

NMN's FDA status: excluded in 2022, reversed in 2025

The regulatory story around NMN is messier than the science. FDA's drug-preclusion rule, section 201(ff)(3)(B) of the Federal Food, Drug, and Cosmetic Act, blocks a substance from being a lawful dietary ingredient once it has been authorized for investigation as a new drug and made the subject of substantial public clinical investigation, unless it was marketed as a supplement first .

In November 2022 the FDA applied that rule to NMN and excluded it from the dietary-supplement definition, on the basis that NMN had been authorized for drug investigation. NMN kept selling anyway, in legal limbo, for about three years .

The FDA reversed course in 2025. In two letters dated September 29, 2025, the agency concluded that NMN is not excluded after all, because it had in fact been marketed as a US dietary supplement before the drug authorization. NMN is again a lawful dietary ingredient, still a New Dietary Ingredient that requires premarket notification. Those letters went to individual companies rather than onto a public FDA page, so the detail here comes from the trade publication NutraIngredients . As of this update (2026-07-08), that is where it stands.

NR NDI status (and the GRAS pathway)

Nicotinamide riboside chloride (the form sold as Niagen / Tru Niagen) reached GRAS no-questions status in 2016 (FDA GRAS Notice GRN 000635) and has been recognized through the New Dietary Ingredient notification process, with NIAGEN NDI notifications acknowledged in 2015 and again in 2018 . That gives NR the longer and less contested regulatory record. ChromaDex and partners published the underlying safety and pharmacokinetic packages behind it .

With the 2022 exclusion of NMN now reversed, the practical gap has narrowed: both are lawful dietary ingredients subject to NDI notification, but NR carries the cleaner paper trail. The trial-evidence base for the two is broadly comparable in size and quality.

Surrogate markers vs outcome endpoints

Almost every NMN and NR trial published as of 2026 uses surrogate endpoints: whole-blood or tissue NAD+ levels, NAD-metabolite ratios (NAAD/NAD+, MeNAM), grip strength, walking speed, insulin sensitivity, blood pressure, aerobic capacity. None has used hard outcome endpoints (mortality, cardiovascular events, incident dementia, incident type 2 diabetes) as a primary endpoint over a multi-year follow-up.

Outcome trials cost more and take years, so short surrogate-endpoint pilots are what the field can afford right now. That is also why the longevity and healthspan claims in NAD-precursor marketing run far ahead of the peer-reviewed record. A 2024 systematic review and meta-analysis of 12 NMN trials (513 participants) put it bluntly: NMN raised blood NAD, but most clinically relevant glucose and lipid outcomes were no different from control, and the authors flagged that an exaggeration of the benefits may exist in the field .

Safety and side effects

Adverse-event profiles in NR trials at doses up to 1000 mg/day have been mild and broadly indistinguishable from placebo across multiple trials . NMN trials at 250 to 1200 mg/day report similar tolerability. Mild GI symptoms (nausea, diarrhea, headache) appear in some subjects across both compounds. The flushing and pruritus seen with nicotinic acid at gram doses are not characteristic of NR or NMN at supplement doses.

Long-term safety beyond about 12 months of continuous dosing is not characterized in published trials. Theoretical concerns about NAD-metabolite accumulation, methyl-group depletion (since NAM clearance proceeds via methylation to MeNAM and 1-methylnicotinamide), and support of cancer-cell metabolism remain academic discussions rather than reported clinical signals.

What the trials do not answer

• Whether sustained whole-blood NAD+ elevation translates into reduced age-related disease incidence or mortality.
• Whether NMN and NR are clinically distinguishable in outcomes that matter, despite their different biosynthetic routes.
• Whether tissue-level (not whole-blood) NAD+ is meaningfully changed in skeletal muscle, liver, or brain at the doses studied.
• Whether timing (morning vs evening), pulsatile vs continuous dosing, or combination with other interventions (sirtuin activators, metformin, exercise) changes outcomes.
• Whether NAD precursors interact with cancer-cell metabolism in ways that matter for safety in any specific population.

Regulatory and doping posture

FDA: NR has GRAS no-questions status and acknowledged NDI notifications ; NMN's 2022 drug-exclusion was reversed in September 2025, so it is again a lawful dietary ingredient subject to NDI notification . Neither compound is FDA-approved as a drug. The NIH Office of Dietary Supplements publishes consumer-facing summaries of niacin and NAD-precursor evidence and risk, though its niacin fact sheet still carries the pre-reversal NMN language as of this writing .

WADA: neither NR nor NMN is a named substance on the current WADA Prohibited List . A competing athlete should still confirm status through Global DRO before use, since anti-doping rules turn on the current annual list and NAD precursors have not been singled out either way.

There is no Cochrane systematic review of NR or NMN clinical outcomes. The existing non-Cochrane reviews report the same gap the trials do: surrogate markers move, hard outcomes are unstudied .

Editorial summary

NMN and NR reliably raise NAD+ surrogate markers in humans. The marketing claims about longevity and healthspan run ahead of the trial record, which is short-duration, small-population, and surrogate-endpoint. NR has the longer regulatory record in the US; NMN was in legal limbo until the FDA reversed the exclusion in September 2025. Long-term safety looks tolerable in published trials, but multi-year exposure data are limited.

For research and educational purposes only. Not medical advice.

pepSmart has not commissioned independent clinical review of this article.

More on how we write and source these pieces: Editorial process and contributor disclosure and Sourcing posture.

Spot an error? Email corrections via /about.

Sources: 18 entries, primary canon (FDA, PubMed, NIH ODS, USADA) plus one reputable trade-press summary acknowledged inline, last reviewed 2026-07-08.