Just say you don't enjoy drinking or the effects it has on you. If they persist, just say "I just don't...LIKE alcohol. I like Coke. Want to buy me another round?
"
The truth is, you DON'T (rationally) enjoy drinking, even if your Beastie does....nor do you like the effects (life-ruining consequences) alcohol has on you. So, you're not lying at all. In fact, you're telling the truth to everyone, including yourself!