About 1 in every 6400 hydrogen atoms is deuterium, with a proton and a neutron in the nucleus, not just a proton. It’s not exactly like regular hydrogen: the bond energies are slightly different (different reduced mass).
Since living things rely on finely tuned chemical reactions, even small changes have undesirable effects. Deuterate an animal enough and it becomes sterile: deuterate it more and it dies. Some simple organisms, such as blue-green algae, can survive complete deuteration, but it messes them up – they grow slowly and look funny.
So, you can poison someone with heavy water – but although the authorities probably wouldn’t detect it, it takes a lot, and it’s expensive. Stick to thallium.
However, just because small amounts of of deuterium are nonlethal, doesn’t mean that they’re harmless. They might be: or they might not. Nobody knows, because nobody has ever looked. You could think of deuterium as a source of noise in biological systems – and maybe those systems would work better without that noise.
So, if you can afford it, you should undoubtedly stick to my kind of bottled water – sparkling de-deuterated. We know that there are no advantages to conventional bottled water – it’s just another way that people dispose of excess income – but de-deuterated water might actually be good for you. There is a logical case for a beneficial effect – perhaps most strongly in the most complicated of all biological systems, the brain. Mind you, if we’re talking general marketing, invoking logic is about as useless as tits on a dinosaur, but some of my readers might care.
The other attractive feature of de-deuterated water, is that it’s damned expensive. Regular consumption might run $1000 a day or more, and that’s for cheapskates that don’t shower in it. Regular consumption would mark you out as a truly special person: I can see it becoming the official drink of Davos and the Bohemian Club. There would, of course, be many associated products: de-deuterated 12-year old Glenlivet, de-deuterated Kobe beef, etc.