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Clonidin is an alpha 2 agonist.

Stimulation of alpha 2 receptors will decrease the prejunctional availability of norepinephrine.

When you have less epinephrine you have less alpha 1 receptor stimulation. (that's why clonidin was used in hypertension :))

So the sympathetic effect on the pupil radial muscle which dilates the pupil has stopped.

So what's left is only the parasympathetic affecting M3 receptors on the sphincter muscle and so you get miosis.
 
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