[vladvasiliu]

As others have said, it's not exactly a drug, you "just" need to push yourself to do it regularly for a while. In my case, it's a few weeks.

What I've found happens is that I actually get used to it, and if for some reason have to skip a session, I'll get the feeling that something's missing because I actually "felt like" doing it.

Of course, this doesn work 100% of the time. Occasionally, I really, really, don't want to pump that iron or go for a run. But having developed the habit, it's actually much easier to push through the "don't feel like it" sensation and get my ass out there, "'cause I have to do it". And I'm usually very happy I did when I get back home.