What Is NAD+? The Coenzyme, Why It Declines and What the Evidence Shows

Yes — NAD+ (nicotinamide adenine dinucleotide) is a coenzyme, not a vitamin or a hormone. A coenzyme is a small molecule an enzyme must have on hand to do its job, and NAD+ is the one your cells use to move electrons through energy production and to fuel DNA repair. It is also a substrate, which is the part most explanations skip, and the distinction matters more than it sounds.

Its best-known job is shuttling electrons through the mitochondrial machinery that produces ATP. No NAD+, no electron transport chain. No electron transport chain, no ATP. No ATP, no cell. The cascade doesn't get more fundamental than that.

But NAD+ isn't just a passive carrier. It's also the substrate — the raw material — for two enzyme families that have become the center of serious aging research.

Sirtuins are a family of seven proteins (SIRT1 through SIRT7) that regulate everything from inflammatory signaling to mitochondrial biogenesis to DNA repair. They're NAD+-dependent — every deacetylation reaction they run consumes one NAD+ molecule. When NAD+ is abundant, sirtuins are active. When it's depleted — which increasingly describes the aging cell — they aren't. SIRT1 handles stress response and mitochondrial function. SIRT3 localizes to the mitochondria and fine-tunes the enzymes of the electron transport chain itself. SIRT6 works on chromatin and is involved in telomere maintenance and DNA damage signaling.

PARP-1 is the cell's primary DNA-damage sensor. When a strand breaks, PARP-1 tags the site with ADP-ribose polymers — repair-crew markings — and each ADP-ribose unit costs one NAD+ molecule. A cell under high DNA-damage load (which is what "chronic oxidative stress" means in practice) runs a sustained PARP-1 response, drawing down NAD+, which in turn suppresses sirtuin activity, which impairs mitochondrial function, which generates more oxidative stress. It's a loop, and it's not a pleasant one.

Then there's CD38 — arguably the least glamorous actor in this story and possibly the most important one for understanding why NAD declines with age. CD38 is an enzyme on immune cells that breaks down NAD+ for its own signaling purposes. Like the sirtuins and PARPs, it consumes NAD+ rather than borrowing it; what sets CD38 apart is that it is a high-volume consumer with no repair job attached. CD38 expression is driven up by chronic low-grade inflammation: IL-6, TNF-alpha, IFN-gamma — the cytokine cocktail that tends to accumulate with age, the one researchers have started calling "inflammaging." In mice, CD38 activity rises with age, and animals lacking CD38 don't show the age-related NAD+ decline seen in normal mice (Camacho-Pereira et al., 2016). That's not a human trial. But it is a plausible mechanism for why NAD falls as reliably as it does — not just because synthesis slows down, but because inflammatory demand goes up.

One more piece: NAMPT (nicotinamide phosphoribosyltransferase) is the rate-limiting enzyme in the salvage pathway — the primary route by which your cells recycle NAD+ precursors back into usable NAD+. NAMPT activity declines with age and is suppressed by some of the same inflammatory signals that upregulate CD38. This matters because it means the aging cell faces a double problem: increased consumption and reduced recycling efficiency. Oral precursors like NMN and NR depend on this salvage machinery to convert to NAD+. If NAMPT is already compromised, that conversion is less efficient than it looks in a healthy young adult.

Both, and that is exactly why levels fall. As a coenzyme, NAD+ is recycled - it carries electrons, hands them off, and is used again. As a substrate it is consumed: every reaction a sirtuin, a PARP-1 enzyme or CD38 runs breaks an NAD+ molecule apart, and the cell must rebuild it from the leftover nicotinamide through the salvage pathway.

That second role is the one that drains the pool. A cell under heavy DNA-damage load spends NAD+ faster than it rebuilds it, and the recycling that keeps the coenzyme side running cannot cover the shortfall. This is what makes NAD+ different from a vitamin you simply need enough of: it is not just used, it is used up.

Your body builds NAD+ three ways. The salvage pathway does most of the work: it recycles the nicotinamide left over from spent NAD+ back into new NAD+, with the enzyme NAMPT as the rate-limiting step. The Preiss-Handler pathway starts from dietary niacin (vitamin B3). The de novo pathway builds NAD+ from scratch out of the amino acid tryptophan, the slowest and least efficient of the three.

This is why supplement strategy concentrates on the salvage pathway. NMN and NR are precursors that feed into it, and NAMPT matters so much because it sits at the pathway doing the bulk of the rebuilding. As NAMPT activity declines with age, the main resupply route slows down at the same time that consumption by repair enzymes is going up. Supply falls, demand rises, and the gap is what people are describing when they talk about NAD+ decline.

The honest answer is: meaningfully, and across multiple tissues, over the adult lifespan. The specific percentage varies by tissue, by measurement method, and by individual — so anyone floating a precise universal number is rounding aggressively. What the research describes is a substantial decline between early adulthood and later decades, driven by the compounding pressure of increased consumption (more DNA damage, more inflammatory signaling, upregulated CD38) against a synthesis pathway that becomes less efficient with age.

There's no standard NAD blood panel that tells you you're "low" — no equivalent of a ferritin draw or a 25-OH vitamin D. What there is, is a growing body of mechanistic evidence explaining why the decline matters and a pharmacological question about the most efficient way to address it.

NAD injections are a method of delivering nicotinamide adenine dinucleotide — the coenzyme your cells use to run energy production, DNA repair, and a class of enzymes called sirtuins — into the body. You'll see the term written as "NAD injections" and "NAD+ injections" interchangeably; the "+" refers to the oxidized, active form of the molecule, and that's what's being administered.

The rationale for the injection route is that it skips the gut, where oral precursors (NMN, NR, nicotinamide) have to be absorbed and converted before they become usable NAD+. What it does not do is put intact NAD+ straight inside your cells. NAD+ is a large molecule that is generally thought to be broken down outside the cell into smaller precursors before it is taken up, and the one published human infusion study found that infused NAD+ was cleared from the blood almost immediately (Grant et al., PMID 31572171). So the case for injecting rests largely on pharmacokinetic reasoning, not on head-to-head human trials.

Two main injection formats exist: subcutaneous (SubQ — into the fatty tissue just under the skin) and intramuscular (IM — into muscle tissue). Both are distinct from IV NAD+ infusions, which enter the bloodstream directly over several hours in a clinical setting; SubQ and IM doses are absorbed from under the skin or from muscle. The differences between these routes — speed of absorption, side-effect profile, and practicality — matter more than most guides acknowledge. More on that below.

In the Protocol MD NAD+ protocol, injections are physician-prescribed, compounded at a licensed 503A pharmacy, and dosed by a physician based on your history and clinical picture. Compounded NAD+ is not FDA-approved. It is a prescription, not a supplement or a wellness shot.

What do NAD+ injections actually do? The honest answer is more nuanced than most guides admit. Here's what the mechanism supports — and where the human evidence is thin.

What NAD injections are doing at the cellular level:

The rationale is that raising NAD+ availability would, in theory, do three things at once: (1) support sirtuin-dependent pathways for mitochondrial function and stress response, (2) let PARP-1 run DNA damage repair without draining the rest of the cell's NAD+ pool, and (3) narrow the demand-deficit gap caused by CD38 and declining NAMPT activity.

What people report noticing (personal reports, not trial outcomes):

• Improved energy and reduced fatigue (the change people mention most often)
• Improved cognitive clarity (less "brain fog")
• Faster exercise recovery
• Improved sleep quality

None of these has been confirmed in a controlled trial of NAD+ injections, and each is easily shaped by expectation, sleep and training changes.

What the evidence actually says:

A 2018 randomized, placebo-controlled crossover trial in Nature Communications found that six weeks of oral nicotinamide riboside was well tolerated and raised NAD+ in healthy middle-aged and older adults; 24 people completed it (Martens et al., 2018). That is a biomarker result, not a symptom result. The mechanistic basis is solid. Large, controlled human trials with clinical endpoints for injectable NAD+ specifically — particularly for longevity — don't yet exist.

The honest framing: NAD injections address a real biochemical mechanism. The effects people report have not been confirmed in controlled trials. The human evidence, particularly at longevity endpoints, is still catching up. Anyone presenting this as settled science is overclaiming.

This is where most guides oversimplify. There are three meaningfully different approaches, and the distinctions matter.

Oral supplements: NMN and NR (the "pills")

NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) are the most heavily studied approaches. The logic is sound: your cells convert these precursors into NAD+ via established enzymatic pathways. A 2018 trial in Nature Communications found that oral NR raised blood NAD+ in healthy middle-aged and older adults. The effect was real. It was also a small, six-week study measuring a blood biomarker, which is the part that gets left out of the supplement-industry citation.

The variability is the key word. Conversion efficiency through NAMPT and related enzymes isn't constant — it varies by tissue, by age, and presumably by individual genetics. You're not directly controlling how much NAD+ ends up inside the cells that need it most; you're hoping the conversion machinery runs efficiently enough to get you there.

In theory, the conversion step matters more for older adults whose salvage pathway is less efficient, though that hasn't been tested as a reason to choose one route over another. NAD injections vs pills comes down to the gut: injections remove the gut-absorption variable, but cells still have to take up and rebuild what is delivered, and no human trial has shown injections outperform oral precursors.

IV NAD+ infusions

A 2019 pilot study tracked plasma and urine during a six-hour NAD+ IV infusion (Grant et al., Frontiers in Aging Neuroscience, PMID 31572171). The surprise: plasma NAD+ did not rise at all for the first two hours, because the infused NAD+ was rapidly and completely cleared from the blood. Some NAD+ was then excreted intact in the urine. IV delivery is not as simple as "more in, more inside the cell."

IV infusions enter the bloodstream directly with no absorption step. The trade-off: clinic visits, long infusion times, and a higher cost per session than injections. Infusion clinics report rate-dependent sensations (flushing, mild chest tightness, nausea) at faster speeds that ease when the drip is slowed; a subcutaneous shot has no drip rate, and its usual side effect is injection-site soreness.

Subcutaneous NAD injections

Subcutaneous administration places NAD+ in the tissue just under the skin, where it is absorbed into the blood. You avoid gut breakdown without the clinic time and per-session cost of IV. No published human study has measured how subcutaneous NAD+ is absorbed or how its blood levels compare with IV.

Whether subcutaneous delivery raises NAD+ inside cells more than oral precursors is untested in humans. The pharmacokinetic argument is plausible. The direct comparison between routes has not been done.

You'll see NAD+ sold three ways, and the difference is the delivery route, not the molecule. NMN and NR are oral precursors — raw ingredients your body has to absorb and convert into NAD+. Injectable and IV NAD+ bypass digestion, but no controlled human trial has shown they raise cellular NAD+ more than oral precursors do. On regulation: the FDA said in 2022 that NMN could not be sold as a dietary supplement, then reversed that position on September 29, 2025, when it responded to a citizen petition and concluded NMN is a lawful dietary ingredient (FDA docket FDA-2023-P-0872). Lawful to sell is not the same as proven to work, and supplements are not reviewed by the FDA for effectiveness. Physician-prescribed, 503A-compounded injectable NAD+ is a prescription route with physician oversight; compounded NAD+ is not FDA-approved.

For the full precursor-vs-direct breakdown, see NMN vs NAD+.

Two questions come up about how to inject NAD+: what the process involves, and whether it can be done at home. The short answers are "less complicated than you'd think" and "with the right prescription and training, yes" — but the order of operations matters.

This section covers administration routes at an educational level only. Dosing, technique, and whether any route is appropriate for you is determined by your prescribing physician — not this blog.

Subcutaneous (SubQ) injection

The most common approach for at-home NAD injection protocols. A small-gauge needle delivers the solution into the subcutaneous fat — typically the abdomen, outer thigh, or back of the upper arm. The same sites you'd use for insulin or a GLP-1 pen. The dose is absorbed from under the skin rather than entering a vein, and it needs no clinic setting.

Technique matters: sterile preparation, proper site rotation to avoid localized irritation, and correct angle and depth. These are things a provider walks you through before you go home with a kit. The mechanics are not complicated — subcutaneous injection is one of the most routine self-administration procedures in medicine. The prescription is the gate.

Intramuscular (IM) injection

IM delivers into muscle tissue — typically the deltoid or vastus lateralis. For most injected drugs, absorption from muscle is faster than from under the skin; it hasn't been measured for NAD+. Some protocols use IM; others prefer SubQ for ease of self-administration. Which is appropriate depends on the prescribed formulation and your provider's determination.

What "home administration" actually means

At Protocol MD: after your intake and physician review, if a SubQ protocol is prescribed, you receive compounded NAD+ from a 503A pharmacy with proper labeling, and a physician who has reviewed your history. Ask your care team how self-injection is taught and what follow-up looks like. You are not improvising with an unlabeled vial from a website.

Response varies, nothing about it is guaranteed, and most people notice little in the first week or two. Energy and sleep are the changes people most often describe, and those reports are hard to separate from lifestyle changes and expectation.

For the week-by-week picture and what the evidence does and doesn't support, read NAD+ injections before and after: what to realistically expect.

Honest answer: the mechanism is real, the reported effects have not been confirmed in controlled trials, and the controlled human evidence is limited — particularly at the longevity endpoints the category tends to attract.

What the evidence supports:

• NAD+ declines with age across multiple tissues (reproduced across organisms and measurement methods)
• Oral NAD precursors raise blood NAD+ levels — confirmed in humans (Martens et al., 2018)
• Sirtuin activation requires NAD+ and is associated with longevity-related signaling in preclinical models
• Mitochondrial function is NAD-dependent, and mitochondrial dysfunction is a central feature of metabolic aging
• Injection avoids gut breakdown; the pharmacokinetic argument for it is plausible, but untested head to head in humans

What the evidence doesn't yet support:

• That NAD injections extend human healthspan or lifespan (not tested at scale)
• That injections produce meaningfully superior clinical outcomes versus oral precursors in controlled human trials
• That the subjective improvements reported by patients are attributable to NAD+ alone versus confounders (better protocols, lifestyle changes, placebo effect for engaged health consumers)

So do NAD injections work? The mechanism makes sense, and some people report effects. No large randomized, placebo-controlled trial has shown a specific clinical outcome for injectable NAD+.

Published human studies of NAD+ and its precursors, most of them small and short, have reported mostly mild side effects: injection-site redness or tenderness with SubQ shots, and flushing, chest tightness or nausea during IV infusions that run too fast. There is no well-defined toxic dose for NAD+ in humans, but that reflects how little it has been studied at high doses, not proof that any amount is safe. Pregnancy, active cancer, heart rhythm problems and several medications need a physician review first (see who shouldn't use it, below).

For the full breakdown by route, see NAD+ injection side effects.

There is no universal NAD+ dose. Your prescribing physician sets the amount, route and schedule from your history, medications and response, and this page does not give dosing instructions.

For what the published protocols and studies have used, and what changes a dose, see NAD+ dosage: what the evidence shows.

NAD injection cost depends on route, provider and protocol length: IV sessions are priced per visit, while at-home programs are usually a monthly price. At Protocol MD, every plan's price is published before checkout. Your card is authorized at checkout and charged only if a physician approves your prescription.

For how clinic, IV and at-home pricing compare, see NAD injections near me.

Who tends to ask about it

• Adults 35+ with declining energy, recovery, or cognitive sharpness who've addressed the foundational variables (sleep, training, nutrition) and want a targeted intervention
• Athletes or high-output professionals interested in recovery support, understanding that the human evidence for this use is limited
• Adults focused on healthy aging who want a physician-supervised option as NAD+ naturally declines, without expecting proven longevity effects

Contraindications and cautions

Active malignancy warrants careful discussion with an oncologist before starting — sirtuin and PARP pathways are not trivial biochemical considerations in that context. Pregnancy and breastfeeding are contraindications. Patients on anticoagulants, antihypertensives, insulin, or chemotherapy need a full medication review first; interaction data for injected NAD+ are sparse, which is a reason for caution, not reassurance. Severe hepatic or renal disease requires specialist input.

Anyone with a history of low blood pressure or arrhythmias should flag this at intake. These aren't disqualifying conditions for every patient, but they change the clinical calculus.

There is a gray market for NAD — you can find subcutaneous vials and IV bags from sellers who operate in the same unregulated space as gray-market peptides. The parallel is direct: the molecule might be exactly what the label claims. The sterility, dosing accuracy, and absence of contaminants — those are questions you're answering with trust in a seller who has no accountability structure and no license at stake.

This isn't a morality lecture. It's a quality-of-evidence argument. When you're introducing something directly into subcutaneous tissue, "probably pharmaceutical-grade" is a strange standard. Products sold through that channel carry no requirement for sterility, purity or accurate dosing — the same gap that applies to gray-market peptides, a supply chain this market overlaps with.

At Protocol MD, NAD+ injections are compounded at a licensed 503A pharmacy, prescribed by a US-licensed physician after a proper intake, and dosed for your specific situation. That structure exists because the injection route requires it. The physicians we work with don't prescribe into the dark.

Protocol MD prescribes by telehealth in all 50 U.S. states: a physician licensed in your state reviews your intake, and if NAD+ is appropriate, the compounded prescription ships to you for home SubQ use. A clinic nearby only matters if you want IV infusions.

For how drip clinics, med spas and telehealth compare on cost and oversight, read where to get NAD+ injections near you.

NAD+ is a coenzyme every cell needs for energy metabolism and DNA repair, and its levels fall with age as repair enzymes and CD38 consume it faster than the salvage pathway rebuilds it. That mechanism is well described: NAD+ is the substrate for the sirtuins, PARPs and CD38 that run DNA repair, gene expression and mitochondrial function (Anas et al., Frontiers in Sports and Active Living, 2026), and declining NAD+ is described as a conserved metabolic hallmark of aging (Pei et al., International Journal of Nanomedicine, 2026).

What the research does not yet establish is what any individual should expect from raising it, by any route. Oral precursors raise blood NAD+ in trials; injections and IV infusions have far less human data. Whether NAD+ therapy makes sense for you is a clinical question for a physician evaluation, not self-directed dosing.