An is a drug that mimics or increases a neurotransmitter's effects, whereas an is a drug that blocks a neurotransmitter's effects. agonist/antagonist antagonist/agonist oxytocin/endorphin endorphin/oxytocin

Agonist/antagonist
 Agonist are drugs that bind and activate a receptor with effecting the response.If the effect results in maximum response it is considered as full agonist. However if the drug that binds to the receptor results only into partial efficacy in relation to the full activation of the receptor, this is considered as partial agonist.

Antagonist are drugs that blocks a biologic response of a hormone or another drug. They usually have affinity to the receptors but has no efficacy. This drug will just inhibit the activation of the receptors by the aganist.