Is it okay to have alcoholic beverages while on lunch break?

Some of my co-workers all wanted to go to a mexican resturant for lunch and margaritas. I was a little apprehensive about this b/c I didn't know if it would be grounds for dismissal of employment. I looked thru the employee handbook but I didn't see anything regarding it. Is it okay to have drinks over lunch and go back to work or is just common knowledge that doing so is prohibited?