This summer I spent some days in Moscow and St. Petersburg. There I also visited a friend at his home. We were there with its family and traditional they drunk a lot of vodka during the meal. We as their guests had also to take some glasses but obviously we had enough before our Russian friends ;) But it was very difficult to convince them that it isn't a good idea for us to drink any more vodka. I think we have slightly offended them.
So how can I avoid to drink a lot of vodka if I'm invited without obviously offending my host? I think it is not too difficult if I don't drink at all. Then I can say I don't drink for religious or medical reasons. But what is a good excuse if I already started and want to quit?