I am wondering out of curiosity, if the heroin in the needle was warm would it hit you faster, and if it was colder would it not hit as hard?
Since IV injection of heroin pushes the H straight into your bloodstream, there should be no difference regardless of the relative temperature of the liquid in the needle. You would be able to feel the H as it entered the vein if it were cold, because your blood is obviously running at body temp, and thus you'll be able to tell, but as far as I know, most H users keep it at room temp.
Well I figure since a commonly-known scientific fact is that substances are more or less reactive depending on the temperature of the substance, it should have a greater/lesser effect depending on the temperature of the H. Hot things react quicker than cold things.
I always cold shoot, and it always hits me in about 2 seconds. I can't imagine having an extra second would make a difference. 420