I don't know how old you are, but you could tell them that as you age, you are not metabolizing alcohol like you used to, and have decided to refrain.This actually does happen to a lot of people, and they can't tolerate as much alcohol as they did when they were younger.Some people find they can't tolerate any.You could tell them you have developed allergies (I actually HAVE developed allergies to certain types).Just make sure if you use either of these, you don't care to drink with this crowd again.
I tell people "I'm in training." Haven't had anyone ask what I'm in training for yet
It would be nice if you could just say "No thanks." But I do know there are occasions when people won't take a simple no for an answer. Personally, I will use excuses for a single occasion, but if friends start pressuring me routinely, I either arrange to see them during events when they are not drinking, or find new friends.