Allegator pretty much said it all.
Pure heroin is pretty easy going on the body. However, since what almost 99% of what people use is bought on the streets, you never know what cuts your shooting into your veins. I knew a kid that found out his dope had been cut with rat poison and he only knew because he became very sick and it showed up in the toxicology results. This is another reason not to heat your heroin if you have powder. Some of these cuts are not water soluble, and cooking it will not "boil out the cut" but rather melt it into the solution, and then later solidify in your blood stream which can cause obstruction in your veins.
Also make sure to always use new, clean syringes to insure that you don't catch infections or destroy your veins. The same goes for the water you use, and your cooker and filters. .2 micron filters can actually filter out bacteria which is pretty awsome as well.
Heroin itself is not toxic, but IV drug use can lead to risky behaviour and complications if you don't educate yourself on it. Always test your dope first too, when you get a new batch. I've had habits where I needed to shoot 5 bags to get a proper high, and then I get a new batch, and one bag will send me to the moon.