Dilation, or widening, of the pupils of the eyes is normal in conditions of low light in order to allow more light to reach the retina. Medically, dilation of the pupils is known as mydriasis. Specific medications known as mydriatics are administered to dilate the pupils for ophthalmologic examination. If the pupils are dilated due to an abnormal response, they may remain dilated even in the presence of sunlight or strong light. Injury to the brain and taking certain drugs are common causes of abnormally dilated pupils. Both medications and drugs of abuse can result in dilated pupils. Dilation of the pupil on one side only is also abnormal and may signal a brain injury or other condition that raises the pressure within the cranial cavity (increased intracranial pressure).