Walking around site at Chernobyl today, I was asked quite a strange question by one visiting student; can you drink heavy water, and is it radioactive? I remember how I loved to ask such questions, so here is my answer.
Heavy water is a form of water that contains the hydrogen isotope deuterium. This has the same chemical formula as any other water, with exception that one or both of the hydrogen atoms are the deuterium isotope instead of the regular protium isotope. While the nucleus of a protium atom consists of a single proton, the nucleus of the deuterium atom contains both a proton and a neutron, making heavy water around 10% heavier in mass than the water we all know and drink.
Heavy water is not radioactive, so in those terms would not induce radiation poisoning or cause you any ill effects.
However, after a short period, biochemical reactions in body cells would be affected by the characteristics between the mass of the hydrogen atoms and how hydrogen bonds are formed, disrupting the cells ability to equally divide. Drinking too much heavy water, would produce similar symptoms to radiation poisoning, as both radiation poisoning and heavy water oxidation, damage the cells ability to repair DNA and replicate.
For mammals, replacing 20% of your water with heavy water is survivable (although not recommended); 25% causes sterilization, and about 50% replacement is lethal.
Other species tolerate heavy water better. For example, algae and bacteria can live using 100% heavy water (no regular water).
Therefore, as long as you don’t drink heavy water long-term, it would be okay to drink.