I generally say I have family commitments.
Are they events that are important for your job, i.e networking etc, or purely social?
There are a few things I go to where there's an actual connection to my job, but if its just drinking etc I dont tend to go.
Our tea, at work have drinks the last Friday of the month, which I go to, but thats in work time and no one has more than 1-2 drinks.