Most of the elements in the first four rows of the periodic (other than the noble gases) have some biological role – which means that you need at least a little. In many cases, we know something about the specific molecules involved (like cobalt in vitamin B12). In others, we know that lab animals or plants that are totally deprived of that element suffer, without knowing exactly why. Occasionally another is discovered: last year they found out that bromine is essential in collagen synthesis.
I said deuterium might be bad for you – and that might be so. Important if true. But since its chemical properties are somewhat different from ordinary hydrogen, more likely than not it has a specific biological role, which would mean that you need it. That’s more interesting than possible toxicity – less important, but more interesting. It introduces the possibility of trace isotopes, the analogs of biological trace elements: deuterium is the most likely one, but you might want to check out carbon, nitrogen, and oxygen too.
Of course being necessary does not mean that it can’t be toxic: quite a few necessary trace elements are toxic at higher dosage.
G. N. Lewis, no slouch, wondered about this possibility back in 1934. As yet nobody has systemically looked for a specific biological role for deuterium. Someone should.