You only need to tell them when and if you feel comfortable doing so. I didn't tell anybody other than SR for the first while. And 4+ months later, only my bf and 3 of my family members know. (Two of whom were supportive and one of whom felt a need to explain why people should 'moderate'. I simply ignored the one who explained why people should moderate.)
If it's an issue of what will I tell them when I'm around them and they offer me alcohol, there are a million things you can say, like you said, you can tell them it makes your stomach hurt, tell them you don't want to wake-up hungover, tell them you're on a new diet, tell them you need to drive to the store later, show up with a bottle of juice, gatorade, water, etc. and tell them you already have something to drink. Is the kiddo in the avatar yours? You could tell them you want to set a good example for your child or children. Or simply, 'No thanks.'