I watched my mother die from one of the most painful cancers, bone cancer. She could not move, someone hitting the bed would send her into shocking pain, and riding in a car was excrusiating. Nothing short of a morphine drip eased her pain. (It also gave her hallucinations).
A year later, I went had breast cancer. I hurt like I have never hurt before. I was honest with my doctor. The first few days, I begged for something, then asked to be taken off but realized I still hurt. He said, "Do you really hurt or is it something else?" I realized, I hurt at night and I couldn't sleep. When I told him this, he gave me something to help me sleep at night.
When I was working in ICU last year, I had a patient constantly complain of pain after a very serious surgery. The nurse I was working with really opened my eyes when he said always assess the patient not the condition.
It's not for the nurse to decide how much someone else feels pain, it is however, a nurses job to assess the patient by asking the right questions (like my doctor did), let the doctor know of the assessment, and let the doctor prescribe. If the doctor feels it's warranted based on your assessment, give it.