@Meltinthemiddle when my DS was 17, I went through a similar thing. I had no idea how to deal with it so I involved SS. It was a truly terrible time with drug deals happening though my letterbox, my DS was who knows where. I cracked down, hard. It was just me, him and his younger sibling.
SS were amazing, I can't fault them. I also got the police to put a marker on my house.
It was the worst time of my life tbh but DS engaged, he had his 18th birthday as a deadline to get a job, contribute, stay at home, quit the drugs or SS would remove him and take him to a hostel.
He quit the drugs and the 'friends', got a brilliant full time job just before he turned 18. I'm so proud of him. He had a GF and nearly broke her heart but they're still together, have bought a house.
That was seven years ago. He is now such an amazing person, has a fabulous career, he and GF are getting married next year and I'm so proud. My DSis was disgusted at me coming down hard and said I should let him just 'be' but I knew he was better than that. And he's proved it.