If you care about energy, recovery, or healthy aging, you’ll meet NAD+ fast. Cells use it to turn food into ATP, repair DNA, and run stress-response enzymes. Levels slide with age, which is why raising NAD+ is getting so much attention. A 2025 Nature Aging review pulls together what we know from human trials and where each precursor fits.

The pathways in one minute

Your body makes NAD+ in three routes:

• Salvage (main route in most tissues): recycles nicotinamide (NAM) back to NAD+ via NAMPT. NR → NMN → NAD+; NMN → NAD+.
• Preiss–Handler: uses niacin (NA). 
• De novo: starts from tryptophan through the kynurenine pathway and converges later; slow and tightly regulated

Learn more: What is NAD+ and why is it important for aging?

At-a-glance: NAD+ precursors comparison

Safety and future directions

An important question is: Are NAD+ boosters safe, especially for long-term use? So far, early trials paint a reassuring picture. Both NR and NMN have shown good safety profiles in human studies . For instance, multiple studies report that daily doses of 1,000 mg or more of NR are well tolerated, with no serious side effects . Similarly, a clinical trial of NMN at 1,250 mg per day for 4 weeks found it to be safe and caused no significant adverse effects in healthy men and women . Participants taking these supplements generally have NAD+ levels go up with minimal downsides – at most, some people report mild symptoms like nausea or fatigue, but those are infrequent.

That said, experts caution that we still need more research before NAD+ boosters can be considered proven anti-aging therapies . Most human trials so far have been short (a few weeks to months) and in relatively small groups. It remains to be seen what happens with continuous, long-term NAD+ supplementation over years. Will the benefits hold up or grow? Are there any hidden risks of keeping NAD+ elevated chronically? Large-scale and longer-duration studies are currently in progress to answer these questions .

Another area of ongoing research is identifying the optimal dosing and timing for NAD+ boosters . Future studies may find that certain groups of people (based on age, genetics, or health status) respond better than others. Scientists are also exploring new NAD+-boosting molecules and improved delivery methods. For example, CD38 inhibitors (to reduce NAD+ breakdown) or novel precursors (sometimes called “second-generation” NAD+ boosters) could work alongside NR or NMN for an even bigger effect . Formulations like liposomal NAD+ precursors or sustained-release capsules are being developed to increase bioavailability, meaning more NAD+ gets into cells where it’s needed.

The 7 NAD+ precursors explained

1. Nicotinamide riboside (NR)

NR is a form of vitamin B3 naturally found in trace amounts in milk. In the salvage pathway, NR converts into NMN and then into NAD+.

Human evidence: Multiple clinical trials show NR can significantly raise NAD+ in blood and muscle. In some studies, NAD+ nearly doubled compared to baseline. NR has been tested up to 2,000 mg/day and is generally well tolerated.

Efficacy: High.

Learn more: What is NR?

2. Nicotinamide mononucleotide (NMN)

NMN is one step away from NAD+, making it a direct precursor in the salvage pathway.

Human evidence: Placebo-controlled trials have shown NMN raises NAD+ in blood. Some studies report improved insulin sensitivity in women with prediabetes and better walking speed in older men. Safety data looks promising up to 1,200 mg/day.

Efficacy: High

Learn more: What is NMN?

3. Nicotinamide (NAM)

NAM, also known as niacinamide, is the standard vitamin B3 form found in many multivitamins. It’s recycled into NAD+ via the NAMPT enzyme.

Human evidence: NAM increases NAD+ levels, but very high doses can block sirtuins — enzymes linked to longevity.

Efficacy: Moderate.

4. Nicotinic acid (NA, niacin)

NA is another vitamin B3 form. It feeds NAD+ production through the Preiss–Handler pathway.

Human evidence: NA raises NAD+ but often causes flushing at common doses. It’s also been used for decades at high doses for lipid management under medical supervision.

Efficacy: Moderate.

5. NRH (dihydronicotinamide riboside)

NRH is the reduced form of NR and enters the NAD+ pool quickly.

Human evidence: No published trials yet. In cells and animal models, NRH raises NAD+ more strongly than NR. However, some studies show it can trigger inflammatory gene activity in immune cells.

Efficacy: Potentially more potent than NR, but unproven in humans.

6. NMNH (reduced NMN)

NMNH is the reduced form of NMN. It can funnel into both NAD+ and NADH pools.

Human evidence: None so far. Early preclinical work suggests it may be even more efficient than NMN in raising NAD+ in tissues. Stability and safety in humans remain unclear.

Efficacy: Potentially more potent than NMN, but unproven in humans.

7. Tryptophan

Tryptophan is an essential amino acid found in dietary protein. It supports NAD+ production through the long de novo (kynurenine) pathway.

Human evidence: While tryptophan contributes to NAD+ in the liver, this pathway is slow and tightly regulated, so supplementation doesn’t meaningfully boost NAD+ in most tissues.

Efficacy: Low.

Hallmarks of aging: where NAD+ fits

NAD+ touches several hallmarks: DNA repair (PARPs), epigenetic regulation and mitochondrial function (sirtuins), and inflammation/senescence. CD38 rises with age and breaks down NAD+ (and can metabolize NMN), which helps explain age-related decline. Preserving or restoring NAD+ supports these maintenance systems.

Learn more on the Hallmarks of aging

Buyer’s guide (quick and practical)

Most readers ask, “Which one should I try?” Based on human data today:

• Start with NR or NMN. Both raise NAD+ reliably in people; choose the format you prefer and stay consistent for 8–12 weeks. 
• Niacin (NA) works but expect flushing; talk to a clinician if you have lipid or glucose issues. 
• NAM is fine at typical vitamin doses. Avoid very high chronic doses if your goal is sirtuin-linked benefits. 
• NRH e NMNH look strong in the lab but aren’t ready for routine use without human data.

Conclusion and next steps

Raising NAD+ is a practical way to support cellular energy, repair, and resilience as we age. NR and NMN stand out today for human data; vitamins NA and NAM help with some trade-offs; NRH/NMNH are promising but need trials. Use the table to choose a starting point, then test and track.

For health-conscious individuals, maintaining NAD+ levels might become part of a proactive wellness strategy. Simple steps like regular exercise and a balanced diet can support NAD+ metabolism (for example, exercise boosts the NAD+-producing enzyme NAMPT). On top of that, NAD+ precursor supplements are available for those who want to try this biohack for potential extra support.

Want to learn more about NMN and NR? Read our extend comparison.