Aside from addiction, obviously, are there really any negative effects from occasional use of opiates?

From occasionally using? Not really.

The only thing I realize is that sometimes you cant piss for a few hours (due to the μ-opioid receptor me thinks).

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W.W.S.J.D. ?

i dont think opiates are too good for your liver either

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make crack like dis

Definately correct, especially when they come mixed with APAP (acetominophen) or if you mix them with other drugs (especially alcohol). But for a short term use, or occasionally, this shouldn't be a problem. But still a valid point.

withdrawels are the worst, but occasional use shouldnt be a problem