Drugs affecting the autonomic nervous system (ANS) are classified into several categories. Adrenergic drugs are those that either mimic or interfere with the activity of neurotransmitters secreted by the adrenal medulla—for example, norepinephrine and epinephrine. These drugs stimulate the SNS, either by direct activation of receptors or by promoting the release of receptor-activating catecholamines. In the other hand, adrenergic antagonists block the activity of acetylcholine, norepinephrine, or other neurotransmitters.