My mentor told me that I am not allowed to take my ADHD medication (Adderall) before or during competitions because it counts as a performance enhancing drug, and gives me an advantage over other drivers. Yet I couldn’t find any rule against it. Is there a rule that I am missing? Is it allowed, but frowned upon?
Simply not true.
A drug you are prescribed to take, you should always take. Someone telling you to not do so is ridiculous. You wouldn’t expect someone who is handicapped to have to get out of their wheelchair because sitting is unfair. It is not their right or place to tell you to not take your medication.
I have never heard a rule like this. As far as I’m aware, there are no rules regarding what you can consume before or during a tournament. (coffee comes to mind ;)) However, in some states, robotics is a part of the Statewide Student Athletics Association, which may have a rule against performance-enhancing drugs, and your mentor may have misinterpreted that rule
Either he is messing with you or just does not want you using the meds. There are no rules in VEX about this. If it is legally prescribed… then it is legal.
I would caution you about this as a rule of thumb. I expect it applies here perfectly well, but in many cases it can get you disqualified. If you have to deal with WADA (or similar), just taking something because a doctor prescribed it can get you disqualified and banned. I’ve been in athletic competition with WADA rules and drug testing, and in some of those competitions I’ve seen athletes disqualified for a perfectly legal drug use to treat an illness (back when pseudo ephedrine was common but not necessary in such drugs). In WADA’s case, if the medication is from the prohibited list and there is no option to switch to something not on the prohibited list (they do provide suggestions, but there is no guarantee those will work for a given individual), then you can get a therapeutic use exemption. To get it, you have to provide information for the therapeutic use exemption.
So, check just in case you’re in a situation like what @brendanmcguire described. If so, just submit whatever you need to, and you should be fine. If you’re not in such a situation, you just don’t need to deal with that.
Just to be clear - any rule applying in this individual’s situation is not a Vex or RECF rule. It would be a rule specific to his state, school, or locality.
Can confirm: not this way in Arizona (Robotics is considered an Arizona Interscholastic Association activity rather than a sport, so they recognize we exist but that’s about all)
I take ADHD medication and have for several years, all of which I have competed in VEX. It is allowed, it is not frowned upon and anyone who tells you otherwise can take it up with me and I can get an official ruling to agree with me.
No Olympic, NCAA, or PIAA (for Pennsylvania) rules here! You can take allergy medicine without issue as well.
Please stick with whatever medications your doctor has prescribed.
My mistake, I’ve just seen mentions of robotics in Arizona being ruled by some additional body, and thought that might be the culprit
Well I’d like to be a body that rules Arizona VEX. 
As a referee, I say there is nothing in the rules prohibiting you from taking any prescribed or OTC medication (or even lawful stimulants like coffee, energy drinks, etc.) for the competition, and even then, I certainly am not qualified to make a determination in such a case. Your medication is something that’s between you and your doctor to discuss, prescribe, and take, and I certainly will not mess with that. No one else should either.
I think your mentor is misinterpreting something, but if there’s actually a rule against it, it’s coming from somewhere other than VEX or RECF. I do take ADHD medication myself, and did when I was a competitor in VEX, and no authority relevant to VEX or my school’s robotics program has ever given me problems about it.