What does the medicine Opana treat?

I was on percocept 10/325, 4 times a day. My doctor switch me to Opana because percocept was not working anymore. The Opana 5mg, did nothing for me, but give me a headache. It did not help at all. Now he has put me on Nucynta. I am afraid to take it. I have heard these doctors are giving this Nucynta to get a kick back from the drug companies. I am tired of spending alot of money for these doctors to line their pockets, by perscribing crap that could kill you.