the potentiation, i believe, is caused by the fact that grapefruit juice is somehow a cytochrome p450-3a4 inhibitor. look at the drugs on the list - they include mostly opiates and benzodiazepines (for chemical/structural reasons, not reasons of effect). grapefruit juice shouldn't potentiate GHB because as far as i know GHB is metabolized by liver alcohol dehydrogenase (LADH), right? into CO2 + H2O...
there are LADH inhibitors, for example disulfram (Antabuse, which makes you sick if you drink. stupid, stupid drug), but they're not common, and not in grapefruit juice.
so...
if there's a chemical reason why this happened, let me speculate thus:
grapefruit juice is highly acidic, obviously, cause the stuff is nasty and burns if you get it in your eye. (so will HCl, hence the goggles)
remember the reaction they used to make G?
GBL --NaOH--> NaGHB
well, it's reversible, if you add acid:
NaGHB --H3O+ (any acid)--> --> NaX + GBL + H2O, where X is the conjugate base of the acid used.
so you may have consumed GBL. i believe GBL does have activity, but i think it takes a lot longer to come on and the trip is longer and less intense.
[This message has been edited by Roches (edited 17 August 2001).]