Toxic? No. Chemically, heavy water is the same as light, it is two hydrogens and an oxygen.
The same as the banana you may enjoy is radioactive from Potassium-41. Toxic? No.
You sure about that? This website disagrees:
http://www.straightdope.com/columns/read/2135/is-heavy-water-dangerous
“Despite the fact the light water and heavy water are chemically identical, heavy water is mildly toxic. How can this be? Since heavy water is heavier than normal water, the speed of chemical reactions involving it is altered somewhat, as is the strength of some types of bonds it forms. This affects certain cellular processes, notably mitosis, or cell division, due to the difference in binding energy in the hydrogen bonds needed to make certain proteins. Mouse studies have shown that drinking only heavy water along with normal feed eventually causes degeneration of tissues that need to replenish themselves frequently, and leads to cumulative damage from injuries that don’t heal as quickly. One study likens the effects to those suffered by chemotherapy patients. Heavy water toxicity manifests itself when about 50% of the water in the body has been replaced by D2O. Prolonged heavy water consumption can cause death.”
I read somewhere else that it’s specifically the hydrogen isotope that causes problems. (With other isotopes the proportional difference in mass is too small to matter much.)