People can certainly have paradoxical reactions to medications, but there is really no medical reason why someone would have them to many or all medications. Even just in psychopharmacology the way valium is metabolized, and acts is totally different than zyprexa,prozac etc..
In my experience people with multiple "allergies" to meds have either been managed by their doc poorly with meds and get side-effects or there is something more psychosomatic at work.

Dr.S.