My tried and tested pre opiate cocktail:
I use this every time, except with codeine because its makes codeine weaker.
T-1:30> A tall glass of 100% pure white grapefruit juice, a same sized glass of tonic water (contains quinine), and 400mgs cimetidine (Tagamet HB).
T-0:30> 50mg diphenhydramine (Benadryl, usually two pills). 500mgs Soma (carisprodial), and 2 TUMS (calcium) if you are planning to take your opiates orally.
T-0:00> Personally, I think rectal administration is the way to go, the high is always more intense and last longer compared to oral for me. It also allows you to have a big meal before taking the opiates as the food wont slow absorption like it would if you ate the pills after eating food, and finally if opiates upset your stomach, this will not. The downside is you need more materials (hot water+a needless syringe) and you also need a few minutes alone to administer your dose, unlike orally where you can pop a couple roxi 30's in the middle of a speech and people wouldn't think a thing about it but try squirting opiate water up your ass in a crowded area and it may attract sine unwanted attention.
There are other OTC drugs that work but IMO mixing too many OTC potentiators make the high feel too dirty for me.