Memantine, it would take me a while to properly articulate all of this information, but I think you might get a kick out of doing your own research on the subject anyway, considering this issue applies to you in a pretty intimate way.
A large group of common psychoactive substances are metabolized by the same enzyme system, namely Cytochrome P450 and its various substrates and isozymes. More and more information is coming out all the time regarding differences in enzyme activity. I'm like you though, so we share this.
-I can drink more Alcohol than most people I know and remain coherent.
-I'm barely effected by the drugs Codeine, Hydrocodone, Oxycodone and Tramadol, all of which require a certain degree of metabolization to become active.
-A dosage to get me to sleep of Alprazolam (Xanax) without tolerance would be 3mg - 4mg
-Morphine and Hydromorphone both effect me at relatively normal dosages, although I'd still call myself a heavyweight.
Just to name a few specific instances, this is a real phenomenon. I wouldn't bring it up with a doctor who's not a specialist in the field though. Doctors are pretty deficient in pharmacological knowledge. Anyway, do your research, reference the drugs that work for you and the ones that don't. Which drugs effect you in the same way as your peers and which ones do you require higher dosages of?