Anon has raised an interesting point, despite himself:

D2O is not radioactive and, assuming you could lay your hands on some, it would not be easy to poison yourself with it. But experiments suggest that if you were to consume large amounts over the course of many days, sufficient to replace at least 25% of your body fluid, you would probably succeed in sterilising yourself and damaging your bone marrow.

Heavy water also has this unusual effect:

"D2O is the only “chemical” agent that consistently affects the frequency of circadian oscillations"

http://www.pnas.org/content/70/7/2037.abstract

"Time is what prevents everything from happening at once" - John Wheeler