Which drug is known to cause constricted pupils?

Fentanyl is a powerful opioid that is known to cause pinpoint pupils or constricted pupils. This effect occurs due to the way opioids interact with the body's opioid receptors, which can lead to decreased reactions in the muscles controlling the pupils. In contrast, substances like PCP and ketamine, which are dissociative anesthetics, typically cause dilated pupils. Vapor, depending on its chemical composition, can have various effects but does not consistently lead to constricted pupils as a primary symptom. Fentanyl's specific mechanism as an opioid distinguishes it in this context as a substance that reliably leads to this physiological response.