Oxycodone is an opiate painkiller and its long term usage can cause dependency. Over time, the effect of oxycodone becomes weaker (tolerance) so that the daily dose may be increased in order to feel the same effect. In time, you can no longer feel in good mood without the drug (psychic dependency). Psychoc dependency then turns into a physical dependency, because without the drug you can’t function properly at all. When you stop taking the drug, you feel symptoms of abstinent syndrome (strong pains, vegetative symptoms like nausea, vomiting, etc). Abstinent syndrome is a real horror for the abusers and forces them to do anything to get more drugs only to avoid the abstinent crisis. The strong pain in the joints that you have experienced after stopping oxycodone can be due both to the previous condition (fibromyalgia) and abstinent syndrome. Methadone is also an opiate painkiller but much stronger than oxycodone. It is now possible that you become addicted to methadone instead of oxycodone. I recommend you to ask for expert help from a drug treatment specialist for help with questions about long-term illness and detox options.