Why do people take vitamin C or drink orange juice prior to rolling on MDMA? Correct me if I'm wrong, but mdma is an amphetamine at heart and citric acid decreases absorption of amphetamines in General so why do it,?
It all goes back to the early research on MDMA. Scientists had known for a while that if they gave a rat (or a monkey) a big enough overdose of MDMA the animal would get an odd sort of brain damage. Fairly early on, it became clear that the damage was happening through oxidative stress (some parts of the brain were basically rusting, being eaten away by reactive chemicals that are produced by our own bodies (like hydrogen peroxide.)
Which is why (a lifetime ago) I started recommending that users might want to make sure their bodies had a decent supply of anti-oxidants on hand as a sort of buffer against potential damage. You might say the idea caught on.
Vitamin C (and vitamin E) where two of the first anti-oxidants experimentally tested for protective effects against MDMA neurotoxicity. So, there's some history there, but also some good reasons for it. Vitamin C (for instance) is very effective as an antioxidant, cheap, and readily available in large, high-purity doses. And, as an essential vitamin, it gets actively transported into our brains; you know it will get where it's needed.
Which made it very convenient for our purposes.
Still, you raise an interesting question: Could vitamin C reduce the effects of MDMA?
There's some reason to think it could have at least a small effect, that's true. Vitamin C is normally acidic, and acidic things can reduce amphetamine absorption. On the other hand, I didn't notice anything (even with fairly high doses of vitamin C.) Other people may have had other experiences, of course.
There is non-acidic vitamin C available if you like (typically calcium ascorbate.)