I’ve reported previously (here) on the deutetrabenazine treatment for Huntington’s disease from Israel’s Teva. But the reason it is so effective is because Teva replaced some of the hydrogen atoms with the heavier isotope deuterium, so that more of it can resist stomach acids and reach the intestines.

https://qz.com/950643/austedo-for-huntingtons-disease-teva-pharmaceuticals-has-found-a-nifty-way-to-keep-drugs-in-your-body-for-longer/