If your aim is to reduce her depression, by giving her pleasure of good sex, you can stimulate her by fore play and make her happy without sex. But if you manipulate her by giving drugs, then you are going in the wrong path.Your priority should be to reduce her depression and help her to solve her problem.
There is no such thing as a pill like that. Also it's very selfish of you to be so concerned with yourself and not care at all about your girlfriend's feelings. Family problems can be very difficult and stressful for majority of people. Instead pressuring her into pleasing your selfish desires, you should be comforting her and giving her a safe place to feel supported and loved. I'm sure that if she was posting here she would tell us how sad she is that her family problems are overwhelming her AND her selfish boyfriend still expects her to be ready to have sex whenever he wants. In which case I would encourage her to distance herself from her boyfriend because that's not how to be a loving and caring partner to someone in distress.