According to this CBC article about Rosetta, one of it discoveries was about the water on the comet:
What does that mean: space-berry instead of strawberry flavoured water?
It is referring to the deuterium-to-hydrogen ratio, as described in this article.
If you drank it, could it sustain you, or might it kill you?
According to Wikipedia,
However the water on 67P is not pure D[sub]2[/sub]O, it is only slightly enriched compared to terrestrial water. Earth’s water has about .015% deuterium while 67P has about .053% (cite). So I would think it would be practically impossible to kill yourself by drinking water with the isotopic composition of 67P’s water.
–Mark
It wouldn’t be even close to dangerous. Mammals show no obvious effect at 20% deuteration, so .053% should be quite safe.
Kinetic isotope effects are pretty subtle, even for hydrogen.