What is wrong with her?! Please help?

My friend Rachel is 18 and she's getting weird and acting so differently and it's scaring me. When I saw her a couple weeks ago she was fine and acting the way she always does, but when I saw her the other day she was shaking so bad and so pale you could see all her veins (which was really gross). I asked her about it and she just tells me its her stress level...how its high. She twitches her arms but she seems to act like its normal. I got her to admit that she uses heroin and I just don't know what to do. I'm scared for her, whats the matter with her?