I think a lot of people have difference responses because we all have different body chemistry.
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1 week ago, I end up in the hospital because my sciatica had flared up to a point where I had extreme difficulty in walking and my PM doctor told me they could not see me that day. Well the hospital ended up giving me hydromorphone, and the doctor told me that I would get very sick from it and throw up because I'm partially opioid naive (Although I take tapentadol/nucynta which is a schedule II, its binding affinity is 18 times less than morphine). Well I didn't end up getting sick at all and my stomach was just fine nor did I get any kind of dizzying rush. It ended up making me feel very relaxed and reduced my pain from a 9 to 4. My roomate said it made me extremely social.
Years ago when I was prescribed Darvocet by a doctor who obviously didn't understand my pain, it had a negative reaction and made me dizzy, nautious, and depressed.
People's bodies process medications differently and often narcotics affect more than just MU recoptors....they can also have affects on things like the GABA system. Just something to keep in mind. :)
You mentioned Tramadol, which has an m1 metabolite that activates MU receptors but only at 1/30th the binding power of morphine, so its narcotic activity is significant enough to have a mild effect on pain levels but its narcotic side effects are relatively low due to its potency. Most of tramadol's pain relief comes from its SNRI action so for people that are sensetive to narcotics, medications like tramadol and its more potent cousin tapentadol can be wonderful things.
The flip side of that coin is that a lot of people have trouble tolerating the SNRI effects of tramadol and the NRI effects of tapentadol. It causes anxiety and headaches for some, and tons of relief for others.
Interestingly enough, I have issues using tramadol but my body can tolerate tapentadol better. I think its because my body is sensetive to the seratonin effects in tramadol (which are stronger in tramadol) and it makes me feel really weird for like 12 hours (a bad weird)