NAD+ taken orally is not going to work because it's not absorbed by the gut and also gets quickly broken down in the gut. Therefore, NAD+ can best be delivered intravenously. Transdermal delivery via patches is also an option - at least in theory. The epidermis is difficult to permeate, and ideally, scientific studies would need to be conducted with specific patches to demonstrate that the NAD+ is sufficiently absorbed and also reaches clinical levels in the bloodstream.
It could be far more interesting to take oral NAD boosters (like NMN) daily than getting a NAD shot every few weeks.
When receiving NAD+ intravenously, the question remains if the NAD+ can actually be taken up by the cells. This still needs to be further researched.
In the case NAD+ cannot enter the cells directly, the NAD can be broken down in the bloodstream into specific constituents, of which the effects can cascade down into the cells. For example, NAD+ in the blood is partly broken down into adenosine, which can enter the cells and then increase ATP levels, among other things.
However, some scientists believe there are specific NAD receptors on the cells that could shuttle NAD into the cells:
“Although very high concentrations of NAD have been shown to enter cells, it is not known whether lower, more physiological concentrations are able to be taken up. Here we show that two mammalian cell types are able to transport low NAD concentrations effectively. Furthermore, extracellular application of NAD was able to counteract FK866-induced cell death and restore intracellular NAD(P) levels. We propose that NAD uptake may play a role in physiological NAD homeostasis.”
- Characterization of nad uptake in mammalian cells, Metabolism and Bioenergetics, 2008
We do know that NAD can be taken up in the mitochondria via the Slc25A51 receptor. It could be likely there is also a similar receptor on the cell membranes to take up the NAD into the cells (or at least in some cells). Up to a few years ago, it was also thought that NMN could not be taken up by the cell, but then in 2019 a transporter protein (Slc12a8) was found that could transport NMN from the blood into cells. Often nature is more clever than we think!