The teen star, 19, has apparently been using Clenbuterol, a drug normally used on horses, in a bid to gain a six-pack.
A source close to Justin revealed: “As well as drinking and smoking weed, Justin’s been taking steroids to give himself a six pack.
“His physique had gone from boy-like to muscled in a very short time and his moods have become wildly erratic.”
He was bailed on Wednesday following his arrest and is believed to be staying at a hotel in Miami, which is surrounded by fans and the media.
It has also been reported that his management are urging Justin to check into rehab but that he is refusing to do so.
In a recent interview, Justin’s manager Scooter Braun revealed that he is becoming increasingly hard to control.
Scooter said: “When Justin was younger, it was, ‘Keep him out of trouble, stop him from falling down, protect him as much as you can from anything that can hurt him. ‘
“When I try to do that now, he’s resentful, he pushes away and rebels. What I’ve come to learn is: be there, give the best advice you can, but he has to be allowed to make his own decisions – and his own mistakes.”
Scooter also admitted to staying up most nights until Justin get’s home, saying: “I’m usually up pretty much all night until I know Justin is in. At night is when the trouble comes.”