Why demerol cause dilated pupils?
Demerol causes dilated pupils because it constricts or causes spasm of the smooth muscle (sphincter) around the pupil. When this smooth muscle contracts, the pupils become wider.
See the image (sphincter) at this link.
I hope this is clear to you. Let me know if you have more queries.
The mechanism of action of Demerol is smooth muscle contraction. It causes contraction of all the visceral smooth muscles like intestinal smooth muscle contraction leading to constipation.
In the eye, it causes contraction of the sphincter pupillae. When the muscle contracts due to Demerol, it shortens in length. Thus causing pupillary dilatation.
Hope this is clear to you now.