My ex-girlfriend was on the verge of being a full blown alcoholic.
She would drink fairly often, and she would drink to blackout nearly ever stinking time. She was in her late 20's at the time...not some college kid.
It was all fine and well for a while. I certainly didn't want to be a prude about her having a good time, but she was a mess when she drank. She would sob uncontrollably about some ridiculous little thing that never made any sense. She never actually got behind the wheel, but only because I forced her into my car to drive her home.
She once got pissed at the people she was hanging out with at a bar one night. She started to walk home (an 8 or 9 mile walk). She ended up calling me at 3:00 in the morning from the side of the road to come pick her up.
That one was especially fun. She had told me earlier that she didn't want to hang out with me as previously planned. She said she was tired and wanted to get some rest. Only after finding her sorry ass on the side of the road did I come to find that she had been drinking it up with her ex-husband and that he was one of the people she had gotten pissed at during the night.
There were too many times I had to hold her hair from her face so she could puke her guts out. There were too many times I had to carry her back to the house after she passed out. It was a mess, but it was hard to see it for what it was at the time. Looking back on it, I can't understand why I put up with it for even a moment.
Oddly enough, we didn't break up because of the drinking. She swore off alcohol, and actually stuck to her guns for nearly a year. We broke up for other reasons that became all the more clear as time went on.
My advice: Talk to her about it. Be up front about it. If you're not sure it's a problem, then just discuss it rather than making accusations. Beyond that, map it out for yourself to figure out how things really are. It's so easy to get caught up in something like that without being able to see the bigger picture until you have plenty of distance.
If she becomes combative, then make choices for yourself. You can love a person all you want, but that love won't correct the bigger problem. Keep yourself healthy. If she chooses to join you in the long run, then so be it. Be willing to give her a chance to correct the issue and to be there for her. If she's not willing, then move on. If she's a responsible and occasional drinker, then she may not have a serious problem...but you should still make it a topic of conversation because you are bothered by it, and she is reacting to that.