Glaucoma, whether spontaneous or drug-induced, is caused in nearly all instances by abnormal resistance to outflow of aqueous humor to the veins on the outer surface of the eye. Three types of glaucoma can affect adults: angle-closure glaucoma, open-angle glaucoma, and the secondary glaucomas that result from preexisting ocular diseases. The adverse effects on intraocular pressure produced by systemic drugs principally involve the first two types of glaucoma. An acute attack of angle-closure glaucoma, which may be precipitated by drugs that dilate the pupil, is caused by the periphery of the iris bulging forward to obstruct the trabecular meshwork and, thus, prevent the aqueous from reaching the outflow channels. In open-angle glaucoma, which may be aggravated by anticholinergic and corticosteroid drugs, excessive resistance to outflow is caused by changes within the outflow channels themselves, mainly within the trabecular meshwork, independent of the size of the pupil. The intraocular pressure usually