Prazosin for Veterinary Use

Therapeutic Class: Alpha-1 adrenergic blocker

Species: Dogs and cats

May Be Prescribed For: Congestive heart failure, systemic and pulmonary hypertension, urethral spasm

Prazosin is a selective, competitive alpha-1 adrenergic blocker. It is used in human medicine to treat hypertension, congestive heart failure, pheochromocytoma, benign prostatic hypertrophy, Reynaud’s disease, sleep disturbances associated with post-traumatic stress disorder, and scorpion envenomation. It is used in veterinary medicine to treat congestive heart failure, systemic and pulmonary hypertension in dogs, and urethral spasm in cats and dogs.

Prazosin has activity on alpha-1 adrenergic receptors but not on alpha-2 adrenergic receptors. Because of its selective activity, prazosin reduces peripheral vascular resistance and both arterial and venous blood pressure without eliciting sympathomimetic side-effects. Although selective alpha-1 blockers can cause orthostatic hypotension, they are less likely to do so than the alpha-1/ alpha-2 blockers, such as phentolamine. Prazosin is metabolized by the liver and primarily excreted in the feces.

Prazosin is an older drug that can be particularly helpful in the treatment of hypertension in cases that have not responded adequately to other medications. It is also useful for the treatment of hypertension associated with pheochromocytoma. Prazosin increases cardiac output while decreasing systemic and pulmonary vascular resistance.

Prazosin can be used to relax smooth muscle associated with the internal urethral sphincter in animals with urethral spasm secondary to functional urethral obstruction. In many instances when prazosin is used to treat urethral spasm a skeletal muscle relaxant, such as diazepam, is also indicated.

• Fainting or loss of consciousness associated with postural hypotension, particularly in the first days of treatment.

• Transient dizziness, lethargy, and other CNS signs.

• GI nausea, vomiting, diarrhea, or constipation.

• Prazosin may cause an initial precipitous drop in blood pressure.

• Prazosin should be used with additional caution in dogs that are already experiencing hypotension or are in chronic renal failure.

• It is not known if prazosin crosses the placenta; minimal quantities are found in maternal milk.

• The majority of drug interactions occur with other drugs that may cause or potentiate hypotension, such as beta blockers, sildenafil, verapamil, and nifedipine.

• Clonidine may decrease the effects of prazosin.

• Overdose with prazosin may cause tachycardia, drowsiness, decreased reflexes, fainting, or loss of consciousness. Hypotension and shock should be monitored and treated. Kidney function may need to be supported.

• If the overdose is recognized promptly and the animal is conscious and stable, GI-emptying protocols should be instituted and activated charcoal should be administered.