What is a common long-term drug used for pupil dilation in veterinary practice?

Atropine is frequently utilized in veterinary practice for long-term pupil dilation due to its ability to produce sustained mydriasis (dilation of the pupil). It works as an anticholinergic agent, blocking the action of acetylcholine on the iris sphincter muscle, which leads to the relaxation of the muscle and resultant dilation of the pupil. The effects of atropine can last from several days to even weeks, making it suitable for conditions requiring prolonged pupil dilation, such as in cases of uveitis or when assessing for intraocular pressure.

This contrasts with other medications listed; while phenylephrine and tropicamide can also induce pupil dilation, their effects are typically much shorter in duration. Phenylephrine primarily acts as a sympathomimetic agent, causing short-term mydriasis, and is often utilized for specific diagnostic purposes rather than long-term management. Tropicamide has a quick onset and short duration, usually lasting only a few hours.

Carbachol is primarily used to reduce intraocular pressure and facilitate miosis (pupil constriction), which is in opposition to the desired effect in this context. Therefore, atropine stands out as the most appropriate choice for long-term pupil dilation.