A dark environment and medications such as eye drops and cough medicines are possible causes of dilated pupils, according to Healthgrades. Dilated pupils can also result from biological or chemical poisoning and brain-related neurological conditions and diseases, such as a tumor or stroke.
The pupils serve as openings where light enters the eyes, and their size regularly changes depending on the amount of light entering the eyes, the focusing distance and the person's emotions, explains Healthgrades. In darker environments, the pupils dilate to enable more light to reach the eye.
Dilated pupils can also occur as an intentional or unintentional side effect of certain medications, notes Healthgrades. For instance, eye care experts apply pupil-dilating eye drops to view the retina and other parts found at the back of the eye. Drugs and medications that can inadvertently cause dilated pupils include cold medicines, decongestants, marijuana and cocaine. These drugs lead to temporary dilation.
Both pupils typically dilate to equal sizes if the cause of dilation is the use of biological or chemical substances, says Healthgrades. Neurological conditions affecting the brain involve dilation of one pupil or dilation of pupils to different sizes. In some cases, unusually dilated pupils are fixed pupils, which are pupils that cannot respond to light. Anyone experiencing dilated pupils due to an injury should seek prompt medical care.