What Drugs Cause Dilated Pupils?

There are many signs we can look for that indicate that someone has been using drugs. In law enforcement and in medical communities that deal with substance misuse, the dilation level of the eyes is considered to be a key marker of illicit drug use, and can even shed light on the particular drug that was used.

Dilation of the pupil (mydriasis) or opening of the iris is caused by the activation of two muscle groups in the eye: the iris sphincter and the iris dilator. The body’s parasympathetic nervous system, which controls a person’s autonomic bodily processes when at rest, is what triggers the sphincter response. The sympathetic nervous system, which controls the body’s fight-or-flight response, triggers action in the dilator.

Certain drugs, most commonly psychotropic stimulants, have a large effect on both and cause the pupils to respond by dilating. Drugs can affect the parasympathetic or sympathetic nervous systems individually or in combination, depending on the type of drug taken. This occurs when elements of the drugs affect neurotransmitters in the brain that work in part to control mydriasis, causing the pupils to dilate.