One of the best things you can do for a friend with a serious drug problem is let them know you are there to support them. Let them know you are concerned about their drug use, and encourage them to seek help from a trusted adult—perhaps a teacher, coach, parent, or counselor can help. You can also help by being a strong positive influence - help them get involved in non-drug-using activities like joining a club, playing music, or playing a sport. However, if your friend is becoming a negative influence in your life you might have to step away from the friendship for a while. Teens who feel their friends are a danger to themselves or others should tell a trusted adult right away – it could save lives. NIDA has a great guide for teens who might have a problem with drugs https://www.drugabuse.gov/related-topics/treatment/what-to-do-if-you-have-problem-drugs-teens-young-adults.