Most pain killers won't have any effect on sudden, externally-oriented pain anyway, so I doubt it would matter. Taking a couple of Tylenol before a spanking (yeah I know we're talking about stronger stuff here, but this is for the example) is going to have no effect on how your ass feels while someone's caning the hell out of it. To have any kind of real effect, a person would have to have a shot of novocaine in each butt cheek beforehand.

However, I feel that playing under the influence of any serious narcotic is a no-no. In the same sense that playing under the influence of alcohol or illegal drugs is bad. I just don't recommend it at all. The reason a person feels pain when they're being tortured is because someone is damaging their body. To dull the sensations of the pain also dulls the mind's accurate interpretation of the severity of the damage. Fooling your body into not thinking you have a hemorrage doesn't mean you really don't have one. I'd just take Nanacy's advice and "Just say no."