Which substance is least likely to cause pupil dilation?

Ketamine is known primarily as a dissociative anesthetic and has unique effects on the body compared to other drugs. While some substances, like stimulants or hallucinogens, typically lead to pupil dilation due to their activation of the sympathetic nervous system, ketamine often produces the opposite effect, particularly at certain dosages.

Ketamine can lead to a range of responses, including pupil constriction (miosis) or having no significant effect on pupil size in some cases, especially in lower doses. This aligns with its profile as a controlled substance, where it causes sedation and altered states of consciousness, rather than overstimulation or heightened alertness that would typically cause dilated pupils.

In contrast, substances like MDMA and other stimulants are well known for producing pupil dilation, therefore making ketamine the least likely to cause this effect among the substances listed.