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Pharmacologic dilation of the pupil happens when the balance shifts toward sympathetic stimulation or away from parasympathetic constriction. Drugs that cause dilation either activate the sympathetic pathways (alpha-adrenergic agonists) or block the parasympathetic input (muscarinic antagonists). Over-use of Visine contains an alpha-adrenergic decongestant, which can promote dilation by stimulating the iris dilator muscle. Tropicamide and the scopolamine patch are muscarinic antagonists that prevent the sphincter pupillae from constricting, also leading to dilation. Pilocarpine, on the other hand, is a cholinergic agonist that stimulates the sphincter pupillae, causing miosis. Since pilocarpine causes constriction rather than dilation, it is least likely to cause pharmacologic dilation.