Several drugs can cause your pupils to become dilated, or bigger. These include stimulants like cocaine, amphetamines, methamphetamine, and ecstasy (MDMA), as well as hallucinogens like LSD, psilocybin (magic mushrooms), and mescaline. Some antidepressants (like SSRIs), benzodiazepines, antihistamines, and certain prescribed stimulants also cause pupil dilation. This happens because these substances affect the muscles in the eye and the nervous system, often through the body's adrenaline (fight-or-flight) response or neurotransmitter effects, which relax the muscles controlling pupil size to let in more light.

Conversely, opioids like heroin and fentanyl usually cause pupils to constrict (get smaller), which is the opposite effect.