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Actually, your pupils dilate with many pleasurable sensations: this is one reason why women draw attention to their eyes with makeup. The idea goes that sexual arousal would cause eyes to dilate, as well as cheeks and lips to flush: all of these are mirrored by modern cosmetics.

It is not known whether the pleasure-induced pupil dilation is a side-effect, a social signal, or an adaptation for receiving as much sensory input as possible.

Many drugs which induce pleasure (often via serotonin signalling pathways in the brain: ecstasy operates this way, for example) will therefore also induce pupil expansion.

Historically, the drug atropine (from the plant "belladonna" - meaning "beautiful woman") was used by women to intentionally dilate their pupils: it is a muscle relaxant, and caused the pupils to open when administered as an eyedrop. This served the same purpose as modern makeup ... but belladonna's other name is "deadly nightshade", because too much atropine will kill you by paralysing your lungs and heart.

Like atropine, some recreational drugs have muscle-relaxing properties (amyl nitrate "poppers" for example will do this), and this is why they dilate pupils.

eye pupil dilate upon using of some drugs as atropine as it acts on M3 receptors on the ciliary muscles of eyes (mydriasis) blocking all the muscarinic receptor on it

other drugs as opioids cause constriction or pinpoint miosis by acting on the receptor stimulating them

Only with relaxants - like canabis.

Opiates have the opposite effect - reduction of the pupil,

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