I think Mainline covered this pretty well, but I will add one thing. The effects of heroin
, for most people, last anywhere from 4-6 hours. If you can get 12 hours out of a single shot
of heroin, consider yourself damn lucky. You get twice as much out of your dope as a normal user, and an immeasurable amount more out of your dope than an addict
... and yet you're trying to extend it? If you ask me, that's just being greedy.
Mainline also makes a very good point that this is a fast-track to either overdosing or physical dependence. Though, the psychological dependence will set in first and it's just as hard to break. I hope your use always stays at the point you can get 12 hours out of one shot, for that would mean you were able to keep it under control.
In addition, in my experience that second dose is never what you want it to be if you're already high. You think it's gonna be great and really add to the experience, but it almost never does. If anything, like stated, it just ups your chances of adverse effects. More often than not, I find it ends up being a complete waste and better kept until the next day or longer.