Does a NAD+ infusion do something that an NMN pill does not?

There is no direct comparative study that has tested a NAD+ infusion and an oral NMN pill in the same people using the same outcome measures. Any claim about what an infusion 'does additionally' therefore cannot be substantiated on the basis of available research.

What oral NMN does do has by now been mapped out reasonably well. In a randomised double-blind study in healthy middle-aged adults, blood NAD+ concentrations rose measurably and in a dose-dependent manner at 300, 600 and 900 mg per day, with the greatest increase at 600 and 900 mg. That is better than nothing, but researchers also note that the effects in humans are smaller than animal studies would lead one to expect. The pill raises NAD+ in the blood, but whether that also sufficiently raises NAD+ levels in the heart, muscles, brain or liver remains unclear. In people with obesity and a fatty liver, for example, clinical effects have so far been modest.

Intravenous administration of NADH (a related molecule, not the same as NAD+) has been studied in 15 Parkinson's disease patients. That small study without a control group showed an improvement in motor symptoms and higher levodopa availability in the blood. However, this says nothing about NAD+ infusions in healthy people or in ageing people without Parkinson's disease, and the strength of evidence from that single study is too weak for firm conclusions.

Theory suggests that an infusion bypasses the intestinal barrier and delivers direct peak concentrations in the blood, but whether those peak levels reach target tissues more effectively than a pill has not been demonstrated. As long as there is no comparative study, an infusion is primarily more expensive and more invasive, not demonstrably more effective.