As Oakman says, no-one really knows.
If you are asking what the maximum effective dosage is, one clue might be that the NAD+ clinics that use slow drip IV mostly use around 1 gram per day.
But we don't know what % of NAD+ makes it into bloodstream by sublingual use.
If it is around 25 to 35% that some other sublingual drugs achieve, that might mean you could use up to 3 grams a day - not cheap!
I don't think any mfg would ever recommend that much, as it would greatly dampen the marketing potential to tell customers they need to spend so much for max effect. Also, clinics have doctors monitor the patients/customers at clinics to ensure safety at the higher dosages.
On the other hand, the 70% or so that does not absorb immediately to blood doesn't get totally wasted - it would be digested to NAM in the stomach, just like anything else you take by capsule. It would surely have much less benefit that way, but not totally useless.
NADH has been sold on Amazon for years. It has good reviews - actually better than Niagen on average. I don't understand why/how that is, since it is NADH, when research shows the benefit comes from increasing ration of NAD+ to NADH. So taking NADH supplements would seem to be the opposite of what we want. But perhaps it has a noticeable effect on mood as the reviewers say, so they like it even if it might actually be bad for most other. things.
I think the overall publicity to NAD+ precursors is one factor for the sudden/increased interest. Also, the success of NMN powder and tabs for sublingual use has made ABN, RealNAD, and others realize the sublingual NAD+ would also have appeal, and bypassing the stomach would make it much more effective than capsules, at a fraction of the cost of NAD+ IV clinics.
Edited by able, 15 December 2018 - 03:29 PM.