ok, i have 'eaten the cow' [as we russians say] on this matter and I have the following to say .
YES, grapefruit juice MAY potentiate the effects of alprazolam [and I recall from what my prof. told me, all of the triazolo analog benzos blah blah blah etc], But I do not care for studies or tweed-suited professors. So I tried myself.
Certain brands of juice work better than others. The juice must be yellow not pink. It must be fresh, not from concentrate, it must have no extra calclium additives [don't know why...ask k'DoutinAZ' he must somehow know]/ Drink one big tall glass 30mins before ingestion of alprazolam...and then take the alprazolam with the juice - swallow it, do not keep it under your tongue [again don't ask me why, I'm not a biochem major]. Have another glass about 20 mins after all this [take 50-100mg hydroxyzine to avoid excessive trips to the bathroom]. It works. I have done this 3 times. 3 times normal ingestion on empty stomach at my usual wake-up time - 2mg. And then thrice during my normal wake up time with the grapefruit juice method [no hydroxyzine involved..that was just an addendum from personal experience].
Although grapefruit juice CAN, and in my case, DID potentiate effects, it gave more of a sedative effect, rather than a giddy, lightheaded xanax effect that we all love. It makes the drug stay in your system longer [for me, 3-5 hours longer than usual]
I have tried the same with diazepam [no difference in effect at 15mg], midazolam, bromazepam, temazepam, lorazepam, nitrazepam, and flunitrazepam [yes, I have a 'history'...and none of it worked. It only works on alprazolam.
PS. My suggestion, instead of ingesting pints of juice until you gag, why not just have a little shot of brandy with your pill? It's the best feeling ever....why go through all this garbage with the graprefruit juice. If someone told you that beet juice was a potentiator...I fear to think.