NAD+ Therapy: IV vs Supplements vs Precursors

Why the Molecule Cannot Enter Cells

NAD+ is a dinucleotide carrying two negative charges at physiological pH. Cell membranes are lipid bilayers, and large charged molecules do not diffuse across them. No transporter for intact NAD+ import into mammalian cells has been established.

Extracellular NAD+ is instead degraded by ectoenzymes, principally CD38 and CD73, into nicotinamide mononucleotide and then nicotinamide riboside and nicotinamide. Those smaller molecules are taken up and used to rebuild NAD+ inside the cell through the salvage pathway.

The consequence is direct: infused NAD+ acts as a precursor donor after extracellular breakdown, not as NAD+ delivered to the cytoplasm. Any marketing that describes an infusion as putting NAD+ into your cells is describing something the biochemistry does not permit.

CD38 activity is itself relevant to ageing. It increases with age and with inflammatory signalling, and it consumes NAD+ and its precursors, which is one of the proposed reasons tissue NAD+ declines. That makes CD38 inhibition a mechanistically interesting target and one with no consumer-available intervention.

The Three Routes Compared

The niacin column is the one most often omitted from this comparison, and it is awkward for the market. Nicotinic acid raises NAD+ in human tissue effectively and costs a fraction of NMN or NR. The flushing is genuinely unpleasant and is a tolerability problem rather than an efficacy one. Nicotinamide is well tolerated, raises NAD+, and at high doses inhibits sirtuins, which is a reason it is not the default choice in a longevity context.

What the Infusion Experience Actually Involves

The side effects during infusion are not incidental and are worth describing because they are rarely disclosed in advance.

Rapid infusion commonly produces nausea, chest tightness or pressure, abdominal cramping, flushing and a sensation of urgency, which is why protocols run slowly, often over two to four hours. The effects are rate-dependent, and slowing the infusion reduces them.

The reason is instructive. Those symptoms plausibly reflect rapid extracellular NAD+ degradation and purinergic receptor signalling, which is to say the discomfort is a sign of the molecule being broken down outside cells, exactly as the biochemistry predicts. The unpleasantness and the mechanism problem have the same cause.

Beyond tolerability, an intravenous line carries the ordinary risks of infection, phlebitis and infiltration, and infusion clinics vary widely in oversight.

What NAD+ Elevation Has Been Shown to Do

Distinguishing what is established from what is claimed:

Established. Oral NMN and NR raise blood NAD+ measurably. Blood levels are not the same as tissue levels, and the correlation is imperfect.

Reasonably supported. Improvements in some measures in older adults in individual trials, including insulin sensitivity in one specific population and some muscle and functional measures, with inconsistent replication.

Not established. Improved cognition, energy or exercise performance in healthy adults, slowed ageing on any validated measure, or any clinical outcome benefit.

Not studied. Intravenous NAD+ for any of the above. The infusion market has essentially no trial base, which is remarkable given the price point.

The clearest read of the trial literature is that NAD+ elevation is real and its functional consequences in healthy adults are unresolved. That places the whole category as a reasonable experiment with a defined observation period rather than an established intervention, and it makes the most expensive delivery route the hardest to justify.

The Regulatory and Safety Notes

NMN's status changed. The Food and Drug Administration has taken the position that NMN is excluded from the dietary supplement category because it has been investigated as a drug, which has affected its availability in the United States. NR remains available and has an established regulatory notification. This is a legal distinction rather than a safety one.

The cancer question is unresolved. NAD+ supports cell proliferation and DNA repair, and both are relevant to cancer in opposite directions. Some preclinical work suggests NAD+ availability may support the growth of existing tumours. There is no human evidence of harm, and this is a genuine open question that argues against use during active malignancy without oncological input.

Nicotinamide inhibits sirtuins at high doses, which is mechanistically counter to the longevity rationale.

Methyl group consumption. Nicotinamide clearance consumes methyl groups through nicotinamide N-methyltransferase, and high-dose use over long periods has a theoretical effect on methyl donor status. The practical significance is unclear and it is a reason not to assume that higher is better.

A Defensible Approach

If the aim is to raise NAD+, the reasoning that follows from the biochemistry is straightforward and unhelpful to the market.

Address what depletes it first. Excess alcohol, chronic inflammatory load, sustained hyperglycaemia and lack of exercise all affect NAD+ metabolism. Exercise raises NAD+ and the enzymes that salvage it, at no cost.

If supplementing, oral precursors are the rational route. NR has the clearest regulatory position and a reasonable trial base. Niacin is far cheaper and effective at raising NAD+, with flushing as the limiting factor.

Set an observation window. Three months, with a defined set of things you are watching. If nothing you can measure or notice changes, the mechanism story is not a reason to continue.

Intravenous NAD+ is difficult to justify on the biochemistry, on the evidence, on the tolerability or on the cost. Everything an infusion can deliver, it delivers as precursors, which is what an oral precursor delivers at a small fraction of the price without the clinic visit.

That is the whole argument, and it rests on a single fact about membrane permeability that the marketing for this category is built on not mentioning.