Girlfriend Drugs/Alcohol Lie?

I've been with this girl for about 6 months and she has a history of drug and alcohol use.. Just recently she told me she used (coke) but she said she hasn't used in about a year.. then more recently while I was with her at a bar with her friends, she went to the bathroom with some guy and when they came out, I asked her what was going on and she said she was buying for her friend that was with us. I asked her if that was the truth and she told me that she did not do anything, just purchased.. is this a blatant lie? I know drinking is the problem behind a lot of this and she told me she would totally cut back and not drink as much.. she seems fine when she has a couple just at like dinner but when she's out with friends it gets out of hand..which leads to bad things.. and I'm not sure if I believe her when she said she hasn't used in a year.. there have been signs in the past where i think she may be on pills or something else.. will this continue or is there hope?