I think my brother is taking drugs? What should I do, I'm scared?

My brother is 21 years old, he has legal custody of me because our parents died a while back when he was 18. I'm 12 years old about to turn 13, and I think my brother is taking drugs. Three or four times a week he will tell me he is going to the supermarket (around 5:00pm or 6:00pm eastern standard time) and then not be back until the next morning. But when I see him in the morning he looks and acts drunk. I know that he's 21 and that he might just be enjoying his first few years of drinking- but my brother is a responsible person and I know he wouldn't leave me home all night if it was just a decent amount of alcohol he was taking. My aunt is a really big help and is there for me a lot when my brother is away at college. He made some new friends there and I'm worried they might be influencing him to take drugs. The last time he pulled an "all-nighter" I called my aunt and she called his cell phone. She said that when he picked up, he sounded very high and that there were a lot of people hollering and yelling around him, and he hung up after about 15 or 20 seconds. This basically confirmed my worst fears, but I don't know what to do. Do you think I am just overreating or is my brother addicted to drugs? I love my brother so much I don't want anything to happen to him. What should I do I'm scared.