One of the physical effects of heroin is difficulty ejaculating, so following a couple days of use though not nearly enough for physical withdrawals to develop will likely lead to a more premature ejaculation.

If you don't love your girl enough to do heroin and help you cum faster then you know what thats your problem.

As an alternative you could discuss with your doctor the problems you are having with your wang and perhaps switch to a medication with less incidence of side effects such as Trileptal.