I'm on Lexapro (escitalopram) and klonopin. Can I drink one beer?

MAYBE. If you are on medications check with your own physician about using alcohol. Lexapro (escitalopram) is an antidepressant and Klonopin is for anxiety. Alcohol is a depressant and is cross tolerant with Klonopin so it is additive in other words. If you truly drank only one regular size beer once in a while it may be alright but do not drive and do not drink and take the medications at the same time.