Dihydrogen Monoxide can also be very dangerous:

Unfortunately, I am addicted to dihydrogen monoxide. I got hooked at a very young age, and now I simply can't live without it. The withdrawal symptoms are horrible, and if prolonged, could be fatal. Take my advice - if you're not already hooked on the stuff, stay away from it. Once you start, you'll be hooked for the rest of your life. There is no known treatment.