You're probably thinking of getting a speck of a radioactive metal (like Uranium or Plutonium) in your lungs. That gets stuck there, the body has no mechanisms

You're probably thinking of getting a speck of a radioactive metal (like Uranium or Plutonium) in your lungs. That gets stuck there, the body has no mechanisms to eliminate it, and it will keep emitting gamma rays all your life. Tritium is very different. It is an isotope of Hydrogen, and a water molecule containing one atom of Tritium is fairly indistinguishable from any other water molecule. The most important thing is that it has no reason to stick to any other Tritium from that glass that you drank. It will spread throughout your body. Like all other water molecules, it will probably be out of your body in a mater of days. Tritium's half life is 12 years, so the vast, super vast majority of all the Tritium you'd drink with that glass of water will be long gone before it has a chance to decay. But even if some of these atoms decay, their decay is a beta particle (an electron, and if you really care one antineutrino as well, although that antineutrino will probably travel through a few galaxies before it interacts with anything). The beta particle is not very damaging (compare that with the gamma rays emitted by Plutonium). For sure a single one is not damaging at all. Now, in contrast with the speck of the radioactive metal above, this beta particle will not be followed by others like it, because the Tritium that emitted it doesn't have any other Tritiums next to it.