||Heroin, like other opioids and opiates, actually causes pupils to constrict (get smaller),
not dilate (get bigger), after use. This constriction, called miosis, is a sign of opiate inebriation that law enforcement looks for in areas with high heroin and other opioid/opiate abuse.|
Many other drugs--like stimulants and classic psychedelics--produce extreme pupillary dilation, causing the pupils to get larger. This effect is called mydriasis. However, drugs that bind to opioid receptors (as agonists, like heroin) have the opposite effect and produce miosis.
The duration that pupils are constricted is directly affected by the half-life of the drug and its metabolites. Heroin and its metabolites (including morphine, 6-monoacetylmorphine, and the gluconurides) have relatively short durations, resulting in pupil constriction for roughly four to five hours after use.