
Imo it’s not just the drugs but the death of his parents too, remember that this all started with the death of his month, he retreated so bad in his mind that he missed almost weeks of memories, he doesn’t even remember where his mom is. The death of his dad, the drugs and Cheong-rim didn’t help in the slightest as well.

The drugs aren’t the only thing that fucked him up. He’s already mentally unstable but his real decline started with the death of his mom. Remember where he dissociated so bad that he lost weeks worth of memories, that’s also where his hallucinations started. The death of his dad, drugs, the stress with Cheong-rim isn’t helping as well.

He’s most likely going through psychosis and may be schizophrenic. Both can be brought on by trauma, stress, too much or too little mental stimulation, and drug use. In rare cases schizophrenia can be activated by using drugs only a single time, especially if it runs in the family. This is a work of fiction though, so it’s hard to contribute any one thing to this situation. Forgetting Cheong-Rim is definitely strange in this context but I think the basis of Sungkyung’s hallucinations is that he’s stuck reliving his worst moments, he’s in hell, and he doesn’t consider Cheong-Rim to be apart of that hell so therefore he doesn’t exist there. Just some thoughts from a social science student. Cheers!
no fucking way a joint and a few lines destroyed sungkyung memory, like if he was going insane I feel like he wouldn't forget Cheong-Rim of all people