[KOV SPOILER WARNING] He was prescribed them after an accident (shortly after shirase died, he started being even more reckless, and broke his shin jumping into a pool from a rooftop). He had to have surgery, and was put on opiates after the procedure. He was addicted by the time his prescription ended, and in a horrible, grieving place mentally, but he happened to be married to a drug lord. Once he wanted something stronger than the pills, he had access to as much heroin as he wanted. Fyodor gave it to him the first time, because he just wanted Chuuya to feel better. The withdrawals were really getting to him, he was in pain, and Fyodor was in a pretty bad place himself, and they just. Made bad choices, and enabled each other’s bad choices. Fyodor couldn’t help Chuuya once that spiral started, so Chuuya sank, and Fyodor just tried to keep himself afloat.