Annabel,
I used Fentanyl for months, and it worked great. 72 hour time release patches from 15mg up to 300 mg dose rate. (I only got up to 75mg).

Fentanyl is a "long term" opioid pain management narcotic, similar to heroin. It is almost perfect in that it retains it's pain management qualities for a very long time and the patch is an easy way to administer the dose. It does take 24 hours before a full therapuetic level is achieved in the bloodstream. It is hard to overdose (although not impossible) because you would have to physically place more than one patch at a time.

Morphine Sulfate is a "short term", fast acting opioid, typically prescribed for "breaththrough pain" and it takes about 30 minutes for full effect if taken orally in pill form. You would typically use morphine prior to eating and some types of exams, etc.

I was never high when taking the pain meds. They are very potent and require medical supervision or at least a plan to wean off of them. Abruptly stopping them can cause convulsions and/or death.

Constipation is also a serious matter when taking these, or any narcotics, so talk to your nutritionist, stay well hydrated, eat foods with fiber, eat fruit if you can and use stool softeners, like Colace.