analgesics

I have been thinking about psychoactive plant substances like marijuana (from the cannabis plant), opium and its derivatives morphine, heroine and codeine (from the opium poppy). We know what these substances do to the human system, but how did they evolve? What advantage, if any, do these...

I'm wondering about over the counter pain relievers. Which are most effective and safe (not brands necessarily, but types like aspirin, ibuprofen, etc). What about alcohol and analgesics? I've heard some can be harmful with alcohol--are there some analgesics that are safer with booze than others?