This summer, I spent some time in Moscow and St. Petersburg; I also visited a friend and his family. Their custom was to drink a lot of vodka during the meal. We, the guests, also had to drink some vodka; but obviously we'd had enough before our Russian friends. ;) But it was very difficult for us to convince them that it would be unwise for us to drink more vodka. I think, in fact, that we slightly offended them.
If I'm invited to a meal with my hosts, how can I avoid drinking a lot of vodka without offending them? I think it wouldn't be too difficult if I didn't drink at all. Then I could say that I don't drink for religious or medical reasons. But what excuse could I use if I've already started drinking and want to stop?