I agree that drinking events (even if company sponsored) are almost never mandatory.
If this is a new job, all the more reason to firmly set the boundary around your non-drinking Now. If you drink once around new people it will be doubly difficult to not drink the next time.
If you have to go or get fired, then I think what nez said above is spot on. Have several things ready to say if you feel overly-tempted or unable to stay centered. Be ready to leave. The sentences that nez said above are awesome. One thing I've not yet done is lie about my non-drinking. Sometimes I'll just flat out say, "I've already had my fair share," sometimes I'm more discreet but I never lie about it.
No one has any right to have any say over what I put in my body. Period.