Deuterium is toxic (in high concentrations) to multicell animals as it changes the angle of.the hydrogen bonds which is key to cellular replication and enzyme prodcution. However you would have to drink all d2o instead of h2o for about a week to begin to notice (need 25-50% of body water). Blocking cellular replication is similar to what chemotherapy does so would.be like bad chemo...eventually the dose is so large it is not useful Cancer drug.
There is also mentions of dizziness and impact on vestibular system (senses) but not the wiki article does not expand on this and the linked article just mentions nystagmus (involuntary eye movements) https://ui.adsabs.harvard.edu/abs/1974Natur.247..404M/abstract