Sounds interesting as a short story. Something like a VN or a cut scene game should work. As a story dealing with drug, the story should aim to invoke the player's interactions and choices that can make Molly's life worse or better.
There are probably two ways to do this type of game. The first is the exploration of a drug abuses and the possible solutions and pitfalls to her problems. Something like Roy Harper from Teen Titans. The other is the deconstruction or a dark fiction about a teen's recklessness and the implications caused by the society that dooms a woman into a life of drug abuses. You can even throw things like social commentary or horror in there if you're aiming for a hard "M" rating.
Nevertheless, drug abuse is a very sensitive subject. You probably need to do a lot of researches. Many of the issues related to drug simply can't come from Wikipedia. If you know someone who abuse drug or work with doctor or medical teachers, they may help you understanding the issues or give examples how drug users think and act. Additional info can help making the game more realistic and useful for audiences. The ultimate goal should be to give the player an exemplar that the player can guide to meet a good solution.