I can believe that, back then I'm sure and especially now(Bobby Lytes admitted to "quitting" coke himself recently) coke is pretty easy to attain, so I'm sure he likely has put a straw up his nose a few times. With how much people are rapping about it and using it as a quick insult to erratic behavior, it has to be easier than meets the eye to attain it.
But I don't think Justin was super strung out tho, compared to most of his peers anyway. Besides him seeing kinda dead in the eyes at times, his body was still on point and I wouldn't be able to tell he was off anything since he always acted the same, enthusiastic and involved in his scenes.
I think it's honestly best that he got out when he did since it would be sad seeing him deteriorate like how Henrik Sommer did with the drug use and reluctance of help