Key Points
- NR does not cause the skin flushing associated with niacin (nicotinic acid) — it lacks affinity for GPR109A, the receptor responsible for niacin-induced flushing.
- Clinical trials consistently report NR as well-tolerated at doses up to 2,000 mg/day; adverse event rates in most trials are similar between NR and placebo groups.
- Adverse events that have been reported in NR trials include mild nausea, leg cramps, headache, and occasional flushing — though flushing appeared in placebo groups at similar rates in key trials, suggesting it may be unrelated to NR.
Short answer: NR has a favorable safety profile across multiple clinical trials, with no serious adverse events attributed to it in healthy adult populations studied so far. The adverse events that do occur are mild and similar in frequency to placebo groups in most trials — but "well-tolerated" doesn't mean completely side-effect free, and there are specific populations and situations that warrant more caution.
Why NR doesn't cause flushing — but niacin does
One of the most common safety questions about NR comes from its relationship to niacin (nicotinic acid), another form of vitamin B3. High-dose niacin causes a well-known, uncomfortable flushing reaction — warmth, redness, and tingling in the skin — that limits its clinical use even when it's therapeutically effective.
NR does not cause this reaction. The mechanism is specific: niacin-induced flushing occurs through activation of GPR109A, a G-protein-coupled receptor. NR has very low affinity for GPR109A and does not activate it. This has been demonstrated in cell line experiments and confirmed in multiple human trials where no meaningful flushing was observed, including the Conze et al. (2019) 8-week RCT where participants took up to 1,000 mg NR daily without reporting flushing. Nicotinamide (niacinamide), another B3 form, also avoids flushing through a different mechanism — but nicotinamide has its own concerns at high doses, including potential sirtuin inhibition, which NR does not share. See our nicotinamide supplement article for that comparison.
What human clinical trials actually report
Conze et al. (2019, Scientific Reports, PMCID PMC6611812) — 8-week RCT in overweight healthy adults at doses of 100, 300, and 1,000 mg NR daily. No flushing was reported in any dose group. Adverse event rates were not significantly different between NR and placebo groups at any dose. No significant changes in clinical labs (renal function, blood lipids, liver markers) were observed. This is the most comprehensive published safety trial for NR in healthy adults.
Dellinger et al. (2017, npj Aging, PMCID PMC5701244) — 8-week RCT in 120 healthy adults aged 60–80 at 1x and 2x NRPT (NR + pterostilbene) doses. No serious adverse events were reported. The study noted the supplement was safe and well-tolerated at both dose levels through the full trial period.
The long-COVID trial — the important adverse event record (eClinicalMedicine, PMID 41357333) — This 2025 double-blind RCT (n=58) in long-COVID patients provided the most detailed adverse event table of any NR trial published to date. Adverse events considered possibly, probably, or definitely related to NR included: muscle cramps, nausea, bruising, worsening headaches, leg cramps, flushing, rash, and vertigo. One serious adverse event was reported but deemed unrelated to NR. Importantly, this trial enrolled a population with pre-existing health conditions, not healthy adults — the adverse event rate may be higher than in general use. And flushing appeared in the NR group at rates comparable to some placebo group findings in prior trials.
The 12-week NR cardiovascular study (referenced in multiple ClinicalTrials protocol documents) — 30 healthy subjects received 600 mg NR twice daily or placebo for 6 weeks in a crossover design. During the NR period, 14 treatment-emergent adverse events in 7 subjects were reported, all of mild severity: nausea, flushing, leg cramps, and increased bruising. During the placebo period, reported events included headache, skin rash, flushing, fainting, and drowsiness. Two out of three flushing cases occurred during the placebo period, suggesting flushing may have been incidental rather than NR-related. Only 2 subjects dropped out of the study during the NR period.
| Adverse event | Frequency in NR trials | Also seen in placebo groups? | Severity |
|---|---|---|---|
| Nausea | Occasional (mild) | Yes, at similar rates | Mild |
| Flushing / warmth | Rare | Yes — 2 of 3 cases occurred during placebo in one crossover trial | Mild |
| Leg cramps / muscle cramps | Occasional | Not consistently | Mild |
| Headache | Occasional | Yes | Mild |
| Increased bruising | Rare | Appears in some trial reports | Mild |
| Rash / skin changes | Rare | Yes | Mild |
| Serious adverse events | None attributed to NR across healthy adult trials | — | — |
Populations and situations that warrant more caution
The safety data reviewed above comes primarily from healthy adult populations. Several groups warrant additional consideration:
Pregnancy and breastfeeding: No human safety data for NR in pregnancy or lactation exists. Standard guidance is to avoid supplements not specifically studied in these populations unless directed by a healthcare provider.
People with cancer or cancer history: NAD+ is involved in cellular metabolism broadly — some researchers have raised theoretical concerns that NAD+ elevation could support cancer cell metabolism in certain cancer types. This is a plausible hypothesis based on cell biology, not a demonstrated harm in humans taking NR, but it's worth discussing with an oncologist if you have active cancer or recent cancer treatment history.
People taking medications that affect metabolism: NR is metabolized through the same NAD+ biosynthesis pathways as other B3 vitamins. Interactions with chemotherapy agents, certain statins, or medications affecting liver metabolism are theoretically possible. Check with a pharmacist or physician if you take prescription medications.
People with kidney disease: Several NR trials have specifically enrolled people with chronic kidney disease (CKD) to study NR's cardiovascular effects. Preliminary data suggests NR is tolerated in mild to moderate CKD, but this is an area of ongoing research and not a green light for unsupervised use with severe kidney impairment.
What about long-term safety?
The longest published human NR trials run to approximately 24 weeks. One study (the 2025 long-COVID trial) provided NR for up to 20 weeks continuously. No long-term safety signals emerged in published data, but "no published signal in trials up to 6 months" is different from "proven safe over multiple years." Long-term safety data in humans simply doesn't exist yet, and this is an honest limitation worth acknowledging when deciding whether to take NR indefinitely.
The Freeberg et al. 2023 review (J Gerontol A, PMID 37068054) noted this gap explicitly — existing human NR trials have generally been short, small, and not designed to detect rare adverse events that might emerge with longer-term use. That's not a reason to assume long-term harm, but it is a reason to stay current with the literature if you plan to take NR continuously.
FAQ
I've heard NR can raise homocysteine levels — is that true?
High-dose nicotinamide (a different B3 form) has been shown to elevate homocysteine in some trials, which is a cardiovascular risk factor. NR does not appear to share this property — the Conze 2019 trial specifically noted that NR "did not elevate LDL cholesterol or dysregulate 1-carbon metabolism" (the pathway that affects homocysteine). This is one of the key safety advantages of NR over other B3 forms.
Can I take NR with other supplements without worrying about interactions?
NR has been tested in combination with pterostilbene (in NRPT trials) and with resveratrol in various formulations without apparent safety concerns. However, supplement stacking is generally under-studied, and combining multiple NAD+-pathway compounds at high doses introduces theoretical uncertainty. As a principle: more is not always better, and a dose in the clinical trial range (250–1,000 mg/day NR) is appropriate without reason to go higher.
Does NR affect blood labs that my doctor monitors?
In published trials, NR has not significantly altered liver enzymes, kidney function markers, blood cell counts, or lipid panels at doses up to 1,000 mg/day. Minor, transient decreases in hematocrit, hemoglobin, and platelet count have been observed in some pharmacokinetic studies, though these were described as slight and not clinically significant. If you have labs drawn while taking NR, there's no established need to stop it beforehand, but mentioning it to your doctor is reasonable.
Is NR safer than NMN?
NR has a larger published human safety dataset than NMN, simply because it has been studied longer in more trials. NMN's human safety data is growing — multiple trials including the Yi 2023 GeroScience RCT show it to be well-tolerated — but NR's track record is longer. Neither has shown serious safety signals in healthy adults at clinically studied doses.
From Nuvirox
Why we formulated NAD+ Restore
- 500 mg Nicotinamide Riboside Chloride per serving — the form used in published human trials, within the dose range shown to be safe and effective
- Polyphenols studied alongside NAD+ pathways for cellular health support: 150 mg trans-resveratrol (Japanese Knotweed) and 50 mg quercetin (Sophora japonica)
- 10 mg galactomannans from fenugreek to support absorption
- 60-day money-back guarantee — long enough to actually evaluate it the way the research says you should
The bottom line
NR has a genuinely favorable safety profile relative to other B3 vitamins and compared to most supplements in the longevity category. No flushing, no sirtuin inhibition at tested doses, no serious adverse events in healthy adult RCTs, and adverse event rates often statistically indistinguishable from placebo. The adverse events that are reported — occasional nausea, mild leg cramps, rare flushing — are mild and typically transient. The honest caveat is that long-term safety data beyond 6 months doesn't yet exist, and certain populations (cancer history, pregnancy, severe kidney disease) warrant medical consultation before starting. For healthy adults at clinical trial doses (250–1,000 mg/day), the published evidence supports NR as well-tolerated.
- Conze D et al. Safety and Metabolism of Long-term Administration of NIAGEN (Nicotinamide Riboside Chloride) in a Randomized, Double-Blind, Placebo-controlled Clinical Trial of Healthy Overweight Adults. Sci Rep. 2019;9:9772. PMCID: PMC6611812. doi:10.1038/s41598-019-46120-z
- Dellinger RW et al. Repeat dose NRPT increases NAD+ levels in humans safely and sustainably. npj Aging Mech Dis. 2017;3:17. PMCID: PMC5701244. doi:10.1038/s41514-017-0016-9
- Davis HE et al. Effects of nicotinamide riboside on NAD+ levels, cognition, and symptom recovery in long-COVID: a randomized controlled trial. eClinicalMedicine. 2025. PMID: 41357333. doi:10.1016/j.eclinm.2025.100567
- Mehmel M et al. Nicotinamide Riboside — The Current State of Research and Therapeutic Uses. Nutrients. 2020;12(6):1616. PMCID: PMC7352172. doi:10.3390/nu12061616
- Freeberg KA et al. Dietary Supplementation With NAD+-Boosting Compounds in Humans: Current Knowledge and Future Directions. J Gerontol A Biol Sci Med Sci. 2023;78(12):2435-2448. PMID: 37068054. doi:10.1093/gerona/glad106
- Trammell SAJ et al. Nicotinamide riboside is uniquely and orally bioavailable in mice and humans. Nat Commun. 2016;7:12948. PMCID: PMC5062546. doi:10.1038/ncomms12948
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational purposes only and is not a substitute for professional medical advice.