You need to replace half your body mass with heavy water for you to die.

Mammals (for example, rats) given heavy water to drink die after a week, at a time when their body water approaches about 50% deuteration.

https://en.m.wikipedia.org/wiki/Heavy_water

Bacteria are entirely immune to the effects of it , however.

Despite the problems of plants and animals in living with too much deuterium, prokaryotic organisms such as bacteria, which do not have the mitotic problems induced by deuterium, may be grown and propagated in fully deuterated conditions, resulting in replacement of all hydrogen atoms in the bacterial proteins and DNA with the deuterium isotope.