An agonist is a drug which combines with a receptor to initiate a response. Receptors are chemicals naturally found in the body which help to carry the pain message before it gets to the brain. Opoids are a group of pain killer medicines which scramble the pain message before it gets to the brain.
Opoid receptors are like a container.
If you fill the container, you will get an effect. Opoids are classified as full or partial agonists which bind to the receptors to further scramble the pain message. Some examples of full opioid agonists are morphine, Dilaudid, Darvon, Demerol, Percocet, Lortab and Vicodin. These drugs have pain relieving properties but also have side effects such as decreased respiration, constipation, nausea and can cause physical dependence. Partial agonists have a limited intrinsic efficacy, this means that the pain control plateaus as the dose is increased and you may not receive sufficient pain relief.