The part that is not controversial
NAD+ is a coenzyme present in every living cell, central to energy metabolism and to the function of enzymes involved in DNA repair and cellular signalling. None of that is disputed. It is textbook biochemistry.
It is also well documented that tissue NAD+ levels decline with age and under various metabolic stresses.
The leap that gets made too easily
From those two facts, a marketing argument gets constructed: levels decline with age, therefore raising them reverses features of ageing. That inference is not established. A biomarker changing with age does not automatically mean correcting it produces the outcomes people want.
Much of the enthusiasm rests on cell and animal work. That research is genuinely interesting and genuinely preliminary, and the gap between "promising in mice" and "demonstrated in people" is where most nutritional and longevity hypotheses fail.
What people actually report
Reported experiences centre on energy and mental clarity, often within the first few weeks. Those reports are real as reports, and they are also exactly the kind of subjective, highly placebo-sensitive outcome that is hardest to interpret without controls.
Response varies. Some people notice a clear difference, some notice nothing. Neither outcome tells you much about the underlying biology.
Practical considerations
Infusion and injection routes are commonly associated with sensations during and shortly after administration — flushing, chest tightness, nausea — that are usually rate-related and settle quickly. Administration rate matters.
A prescriber should review your history before any NAD+ protocol, as with anything else prescribed.
How to hold the question
The honest position is that the biochemistry is solid, the intervention is plausible, and the human outcome evidence is not yet strong enough to justify confident claims. Anyone telling you otherwise is ahead of the data.
That does not make it unreasonable to try under supervision. It does make it unreasonable to expect a specific outcome.