Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




(for veterinary information only)




1 mg, 2 mg & 5 mg




The muscles of our body can be employed in voluntary activities (like running and playing) as well as in involuntary activities (like intestinal contraction and pupil constriction). Involuntary (automatic) activities are controlled by the autonomic nervous system and the autonomic nervous system is divided into the sympathetic and the parasympathetic nervous systems. The sympathetic system supports the "fight or flight" response while the parasympathetic system supports the "status quo" or normal every day body functions.

The sympathetic system utilizes receptors called alpha and beta receptors to exert its effects. These receptors are located in all the organs controlled by the sympathetic nervous system, some organs using alpha receptors and some using beta receptors. Alpha and beta receptors are further divided into alpha-1 and alpha-2 and beta-1 and beta-2 receptors. These receptors can be enhanced or blocked by medication depending on the effect desired.

Prazosin is an alpha-1 blocker. The sympathetic system is in charge of urine storage (versus urination) and blocking the alpha-1 receptors in the urethral sphincter serves to relax the muscles and dilate the urethra allowing for easier urination.

Alpha-1 receptors are also present in the peripheral blood vessels. Prazosin serves to relax these muscles as well thus allowing for a reduction in blood pressure and dilation of blood vessels (both veins and arteries).



The two scenarios where Prazosin is most commonly used are in cardiovascular disease and in alleviating difficulty in urination.

Urinary Conditions: If a narrowing of the urethra is making urination difficult, the alpha-1 blockade of prazosin may facilitate a more comfortable urination. It is commonly used in cats after an idiopathic cystitis blockage, in patients with bladder or prostate tumors, or patients with spinal disease.

Cardiovascular disease: Prazosin might be used to reduce high blood pressure or even in the management of congestive heart failure.

Prazosin is best given with food (but can be given with or without food) with most dosing regimens including administration 2-3 times daily. If a dose is accidentally skipped, it can be given when it is remembered as long as timing of the next dose is appropriate. Do not double up on the next dose. Store prazosin at room temperature.



Lethargy is the most common side effect but it is generally mild. Humans taking prazosin have reported dizziness. Some animals will raise their 3rd eyelids while on prazosin. This is not of concern but may look odd.

Intestinal effects such as vomiting, diarrhea, constipation or appetite loss can occur. These are generally mitigated or prevented by giving the medication with a small amount of food.

Because one of its main functions is reducing blood pressure, some patients may be weak or tired if their blood pressure drops too low. A dosage adjustment may be necessary. This is usually not serious but could be especially if addition medications are in use which could drop blood pressure (see below).



In humans, tolerance has been a problem with Prazosin (meaning that after a while it does not work any more). Using the diuretic spironolactone concurrently seems to ameliorate this phenomenon.

Some patients require multiple drugs to control blood pressure but it is important to note that hypotensive agents should not be combined without realizing that their effects will likely be additive. Medications that can increase the risk of low blood pressure include: ACE inhibitors, amlodipine, beta blockers, pentoxifylline, sildenafil, and telmisartan.



Prazosin may not be a good choice for patients with pre-existing low blood pressure.

Prazosin is best given with food.

Dogs with the ABCB1 (formerly called the MDR1) mutation may be sensitive to this drug as they are sensitive to many other drugs. Prazosin may not be a good choice for such individuals. Dogs with this mutation are usually of the collie family but individuals may be genetically screened by Washington State University. For more information on testing, visit:


Short version (to help us comply with "Lizzie's Law")

Page posted: 1/19/2018
Page last updated: 10/23/2021