How should I convince my girlfriend not to drink?

I am from India, if that helps to understand the problem.
Both my girlfriend and I don't mind a drink or two. But my problem lies when she goes out with her friends (most of whom I don't know) she has to have a drink.
I have a problem with her going out with them and drinking. Whenever I confronted her, she's tried avoiding the topic. As a result, we have had several heated arguments.