Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.

A constriction response (miosis), is the narrowing of the pupil, which may be caused by scleral buckles or drugs such as opiates/opioids or anti hypertension medications. Constriction of the pupil occurs when the circular muscle, controlled by the parasympathetic nervous system (PSNS), contracts.

A dilation response (mydriasis), is the widening of the pupil and may be caused by adrenaline, anticholinergic agents or drugs such as MDMA, cocaine, amphetamines and some hallucinogenics. Dilation of the pupil occurs when the smooth cells of the radial muscle, controlled by the sympathetic nervous system (SNS), contract.