An<unk>  is a drug which acts as an antagonist of one or more of the<unk>  receptors. Examples include caffeine, theophylline, and theobromine.
Adenosine receptor antagonist
Muscarinic antagonist
Alpha-adrenergic agonist
Beta-adrenergic agonist
Norepinephrine–dopamine disinhibitor
Serotonin receptor agonist
Beta2-adrenergic agonist
Adenosine reuptake inhibitor