Worried about opiod medication

Hello, I've been following the program for 10 years and am enjoying life in a way I never imagined possible. I've been told by my doctor that I need to have my knee replaced and while it will be great to be pain free I am extremely worried about the Morphine based pain relief that I will need.
My weakest leg is definitely BE and I am terrified at the thought of the consequences of this medication. I would be very grateful of any thoughts or suggestions or anything whatsoever from anyone.
The thought of the medication is putting me off having the operation.