Adenosine, a purine antagonist, counteracts the effects of beta-adrenergic stimulation. Thus, its effects are similar to those of the beta-adrenergic and calcium channel blocking agents. It causes sinus bradycardia and slows AV nodal conduction. The drug is used only intravenously, has a rapid onset of action and is rapidly metabolized, resulting in an extremely short half life. For these reasons, it has become first-line therapy for reentrant AV nodal tachycardias.