Even if you've had drinks with them before, there's nothing stopping you from saying "I've decided to stop drinking" or "I'm cutting back on my alcohol intake."
And if they ask why, tell a lengthy story about how the last time you had a drink, it turned out to be drugged, and when you woke up you were in a brothel in Jamaica, in bed with two women, a man, an alpaca, and what appeared to be several half-eaten packets of ramen. And, really, you'd like to keep that as a once-in-a-lifetime thing.