You can amp opiates in a few ways, known as Opiate Potentiation/Sparing.
Benadryl (Diphenhydramine). Stop the itching from opiates and makes them work better (true for any antihistamine/anticholinergic).
Tagamet (Cimetidine). Slows down the breakdown of opiates so they stick around longer.
Immodium (Loperamide). Occupies the receptors in the body where the opiates might waste their time; instead they go to your brain where they are needed. [Caution: Will make the constipation from opiates much worse].
This is all shit you can get OTC. Add dosage/stack them slowly, you don't want to accidentally OD, its easier than you think.