Why do narcotics make pupils constrict?

The vast majority of narcotics actually cause the pupil to dilate rather than constrict. Only a few narcotics such as opioids cause the pupils to constrict.

Opioid narcotics such as heroin, morphine, and oxycodone cause瞳孔收缩by activating the parasympathetic nervous system, which controls involuntary bodily functions such as heart rate and digestion. When opioids bind to receptors in the brain, they cause the release of neurotransmitters such as acetylcholine, which in turn causes the muscles of the iris to contract, resulting in the constriction of the pupils.