Heavy water is oxygen and the deuterium isotope of hydrogen. Here's a look at whether heavy water is radioactive and whether it is safe to drink. //

Just because heavy water isn't radioactive doesn't mean it's completely safe to drink. If you ingested enough heavy water, the biochemical reactions in your cells would be affected by the difference in the mass of the hydrogen atoms and how well they form hydrogen bonds.

You could consume a single glass of heavy water without suffering any major ill effects... //

Theoretically, you'd have to replace 20 to 50% of the regular hydrogen in your body with deuterium to experience symptoms ranging from distressing to catastrophic. For mammals, replacing 20% of the body's water with heavy water is survivable (although not recommended); 25% causes sterilization, and about 50% replacement is lethal.2