“Concern about lethal dosage of prescribed pain medication?”

My partner is permanently disabled due to spine injuries and is currently awaiting a hip replacement. He has been on prescribed pain meds for 8 years. Originally he was never cycled on his Percocet for 7 out of 8 years. He has started with a new pain management dr who cycles his medication to aid in his pain management. I became aware of his misuse of his medication recently and since he is seeking treatment for his abuse. Still, he is still struggling with the prescription meds I was wondering what the lethal limit would be on his pills. He is prescribed 10/325 of Percocet currently. The other day he has taken 18 of them in a 12-hour time span. He has been known to take up to 22 in a day which freaks me out and makes me watch his every breath. I keep telling him today 18 may be okay and tomorrow 18 could kill him. I was wondering if there was any threshold to be aware of. He is currently enrolled in a program to help him and he is actively working on the program but is still struggling.