SwiPA finds that anything over a very small dose of opiates
will still let him get it up, but make it nearly impossible to climax. And by nearly, I mean completely.
As a side note, SwiPA quite enjoys taking a normal dose less than a few minutes before engaging in sexual activity, and climaxing right before or during the come-up. He thinks of it like a race- sometimes he finishes, and sometimes he is, er... disqualified?
When he can pull it off, he says it tends to kick-start the endorphin rush. SwiPA has only tried this with oral injestion, but any other delayed-onset method of administration would work theoretically.