An agonist is a chemical that binds to a receptor of a cell and triggers a response by the cell. An agonist often mimics the action of a naturally occurring substance.
A bad guy if you are talking about human characters in a story. But if you are talking about muscles, it is the one that acts to oppose the action of the prime mover, or agonist. For example: The biceps and triceps have an agonsit/antagonist relationship. When you flex your elbow (pretend you are Arnold Schwarzenegger and you want to 'bulge you biceps'). When you do that your triceps relaxes to allow the biceps to contract. But, in actuality, both are contracting, only the biceps are getting shorter and the triceps are getting longer. In other words, if you did not have the triceps to control and mitigate the movement of the biceps, you'd end up punching yourself in the face because there was no way to control the biceps shortening and flexing the elbow until you KO'd yourself. In other words, the agonist, or prime mover is the gas pedal and the antagonist is like the brakes. You need them both to control movement in the body.
Methadone will block any effect that the Oxycontin would give. First of, the RX you are referring to is Oxycontin NOT Oxycontin as refered above. As a RN and nurse anesthetist for 25 years I can tell you with 100% accuracy that the combination of these 2 powerful narcotics will lead to respiratory arrest and death UNLESS you are being treated medically for breakthrough pain-e.g. end stage cancer. Also, as law student I find it offensive that non-medically trained people are giving out information that can KILL. When you take Methadone and Oxycontin which is time released oxycodone by the way will land you on the Coroner's table. Methadone is a full agonsit synthetic narcotic and it does NOT block ANY opiates it potentiates or increases the effects of both drugs and leads to DEATH. Whatever imbecile wrote this has no business giving medical advise especially regarding powerful narcotics. NEVER EVER combine these 2 drugs unless you have cancer with high pain tolerance and under medical supervision or the end result is DEATH period. Suboxone and Narcan block Oxycontin, methadone and ALL opiates. This person got it totally backwards and hopefully did not harm anyone due to this most ignorant reckless advice that could lead to lawsuit. In the future please use medical professionals to answer these questions. If someone took this advice they could be dead by now...unbelievable. Respectfully, P. Anthony Parks RN-Legal Nurse Consultant