Are There Drops to Reverse Eye Dilation? Q. Why do doctors dilate your eyes for an exam, and are there any drops to reverse the dilation? A. Doctors use a class of drugs called mydriatics to widen the pupil, enabling them to see the retina and optic nerve. Depending on the drug, mydriatics work by paralyzing the muscle that makes the pupil smaller or by stimulating the muscle that dilates the iris. There is an antidilation drug, dapiprazole, that blocks stimulation of the muscle that widens the iris. In a 1994 clinical study, published in the journal Optometry and Vision Science, the drug was tested in one eye in 30 subjects who had been given drops containing the dilation drug tropicamide. Average recovery time was significantly less in the treated eyes, and comfortable reading ability returned after about 43 minutes with dapiprazole, compared with 66 minutes without it. All subjects had red eyes that persisted for a 180-minute observation period. Dr. Stanley Chang, chairman of ophthalmology at NewYork-Presbyterian Hospital/Columbia University Medical Center, said that dapiprazole was popular with his patients and that it usually brought the pupils down in about an hour. But he added that the drug was now off the market in this country; he said he was not sure why. "There are no other compounds available to reverse dilation at this point," he said. However, the effect of most dilating drops wears off in about two hours, and the patient is able to resume reading. Readers may submit questions by mail to Question, Science Times, The New York Times, 620 Eighth Avenue, New York, N.Y. 10018, or by e-mail to question@nytimes.com.