There isn't one. A lot of people follow David Sinclair and take 1gm a day. This is a lot and it's expensive. Some take more - this is mainly my fellow 0ver 50 awesome folks. Anyone much younger taking this amount without medical advice might be doing themselves a disservice, as there is no long term studies and the suggestion that NMN may, emphasis on may, assist existing cancers to grow. Best is to take the lowest dose that has a positive effect on you. One research study concluded that people over 60 could benefit from a lower dose of 250mg a day. Many people jump in too quickly and try taking 500gm on their first day, then get side effects and decide it's devil's poison. I initially had side effects on 300mg in my first week. I backed off and titrated up slowly from under 100gm to now, 6 months later, about 350-450 a day. I don't seem to need more. If you take too much and you are not used to it, people report effects such as headache, feeling buzzed or wired, insomnia, stomach upset, heart palpitations (the last could also be anxiety from taking a new substance but can't ignore it). I had insomnia for a month on 200mg a day but weirdly, it didn't make me really tired. If it's taken later in the day it can cause insomnia, too. I also felt very wired, as if on speed (never had it but it felt like this would be what it feels like) which was also unpleasant, so I needed to back off and start again. I have read anecdotes of this side effect, too. With NMN going in boots and all isn't a good idea, even Sinclair says to increase dose slowly. The best way is to start small, say 100mg, sit on that for a week, then keep increasing until you get to a dose where you really notice it making a difference and that you can financially afford to maintain. For some people this can take weeks, others like me feel effects almost immediately. However if you're younger, it may not do anything for you and may even be harmful down the track, we don't know yet. I really wish younger people would stop thinking NMN is going to make them bullet proof or give them some edge and will solve their problems. You take iron if you have an iron deficiency, if you don't have an iron deficiency, taking it won't make you stronger and certainly can harm you. Take NMN because your NAD is depleted and you have a deficiency, which is certainly the case if you are older or have certain illnesses, taking it when your NAD levels are normal may not do anything for you and may harm you in the longer term - we simply don't know. And someone is going to come along and crush my analogy but that's ok, I'm hoping to get the message across to younger folk that they probably don't need it and would be better off making lifestyle changes first, diet/exercise/sleep including getting help for feeling low, if they are feeling crappy. If you are over 40, none of this applies to you, you can probably take NMN and get benefits.