Intramuscular injection of chlorpromazine decreases intraocular pressure by lowering systemic blood pressure.

Intramuscular injection of chlorpromazine in rabbits caused a significant decrease in intraocular pressure (IOP). The dose-response curve was generated. The threshold dose was 0.1 mg/kg (approximately 0.35 mg per rabbit) and 10 mg/kg chlorpromazine produced the maximal response which lasted for several hours. This decrease in IOP was not due to the release of pituitary prolactin by a central dopaminergic-2 antagonistic activity of chlorpromazine. Intracerebroventricular (i.c.v.) injection of chlorpromazine also caused a decrease in IOP. The threshold dose was 0.33 mg per rabbit. This high i.c.v. threshold dose indicates that no direct central mechanism is responsible for the decrease of IOP after an i.m. injection of chlorpromazine. Intravitreal injection of 0.35 mg chlorpromazine caused significant miosis without any change in IOP. It appears that direct mechanisms of chlorpromazine in ocular tissues do not decrease IOP. After an i.m. injection of 0.1 or 10 mg/kg chlorpromazine, systemic blood pressure (BP) was lowered in a similar pattern as the decrease in IOP. Neither IOP nor BP was affected by an i.m. injection of a subthreshold chlorpromazine dose of 0.01 mg/kg. These observations suggest that the decrease in IOP after an i.m. injection of chlorpromazine is mainly due to the decrease of BP.