Which medication is most likely to cause mydriasis (dilated pupils)?

Mydriasis, or dilated pupils, can be a side effect of certain medications, and antihistamines are known to cause this effect due to their anticholinergic properties. Antihistamines block the action of histamine at H1 receptors, and many of them also have significant anticholinergic activity, which can lead to a decrease in the parasympathetic stimulation that normally keeps the pupils constricted. When this parasympathetic tone is reduced, the pupils tend to dilate.

Other medication classes, such as antibiotics, analgesics, and bronchodilators, are less commonly associated with causing mydriasis. Antibiotics have a wide range of mechanisms depending on their class but do not typically result in pupil dilation. Analgesics primarily focus on pain relief and are not known to cause significant changes in pupil size. While some bronchodilators can have sympathomimetic effects that could potentially lead to pupil dilation, it is not a prominent or characteristic effect of this class compared to antihistamines. Thus, antihistamines are the class most directly linked to the occurrence of mydriasis.