I'm around 5'6" and before I started drinking, I usually hovered around 125 lbs. Then when I started drinking a lot, I shot up to nearly 170 lbs over the course of a couple of years (NOT advised). I've quit drinking now, but I'm eating plenty of protein (eggs, chicken, soy, whey protein shakes, occasional steak sandwich, etc). I remember at one point in high school I was down to almost 110 lbs. It can be very uncomfortable and it never feels healthy. If you want to get bigger, you need to eat big. Definitely stay away from drugs, especially uppers. As it's been said in this thread, those will burn off those pounds fast.
If you want to gain muscle mass, then you're looking at heavy weights and low reps, with a good fast-acting protein shake immediately after your workout. You need to lift heavy, but be careful not to injure yourself. Challenge yourself, but pace yourself. But if you pick up a light weight and lift it for 100 reps, you're not going to gain much size at all.
Don't make the mistake of digging into fast food and junk food to put on pounds, because you're just going to feel like shit and it won't do a whole lot for you. You clearly have a fast metabolism. You need proteins and you need a steady and focused diet so you can arrive at a comfortable weight and size. Chicken breast, eggs, nuts, and beans are all good. Make sure to throw in plenty of vegetables (and/or pick up some V8 Juice) and eat an apple or an orange with your breakfast. Sleep well and take rest days from your workouts.
ALWAYS keep your doctor up to speed on your plans and how things are going, though. I'm pretty sure none of us are doctors (I'm sure as hell not). If anybody disagrees with any of this advice, please respond with your input because I'm not always right. I'm just saying that I've seen this stuff work.