It depends somewhat on where the pain originates.
An elderly friend was suffering from terminal spinal cancer. At times, that caused a lot of pain, and it was not relieved by weaker drugs. Something about the pain being in/near the spinal cord, a direct pathway to the brain. Eventually, to relieve the pain. she had to be given morphine in very large doses – ones that her doctor said could have been fatal to a patient not used to morphine. On bad days, the dose needed often left her groggy and falling asleep.
The dose used was large enough that there was a fair risk that it could depress her breathing, possibly enough to be fatal. But her doctor said he had discussed that with her, and she was willing to take the risk. As she said, “if that happens, it’s not a bad way to go. And I’m prepared to go anyway. If I’m racked up with pain, or so doped up I just sleep, I’m no use to anyone anyway.”
She lasted several months, receiving a massive amount of morphine 2-3 times per week.