Brand Name: Minipress

Available as: 1 mg, 2 mg & 5 mg capsules (frequently must be compounded for smaller animals)

Background

Our muscles can be used in voluntary activities such as running and walking as well as in involuntary activities such as intestinal contraction and pupil constriction.  Involuntary (automatic) activities are controlled by the autonomic nervous system, which is divided into the sympathetic and the parasympathetic nervous systems. The sympathetic system supports the fight or flight response while the parasympathetic system supports normal, every day body functions.

The sympathetic system uses 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). Blocking the alpha-1 receptors in the urethral sphincter serves to relax the muscles and dilate the urethra to allow for easier urination.

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

How this Medication is Used

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 make it more comfortable. It has been commonly used in cats after an idiopathic cystitis blockage, in patients with bladder or prostate tumors, or patients with spinal disease. The effectiveness of this medication for recurrent feline urethral obstructions has come into question, and use for urethral relaxation in cats is considered controversial.

Cardiovascular disease: prazosin might be used to reduce high blood pressure or even to manage congestive heart failure.

Prazosin is best given with food (but can be given with or without food) with most dosing regimens giving it 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.

Side Effects

Lethargy is the most common side effect, but it is generally mild. People taking prazosin have reported dizziness. Some animals will raise their third 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 effects are generally eased or prevented by giving the medication with a small amount of food.

Racing heart rate, hyperactivity, or increased body temperature are side effects that should be reported to your veterinarian. This is usually not serious but could be, especially if additional medications are being used that could drop blood pressure (see below).

Interactions with other Drugs

In humans, tolerance has been a problem with prazosin, meaning that after a while it doesn’t work anymore. 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.

Concerns and Cautions

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

Dogs with the ABCB1 (formerly called the MDR1) mutation may be sensitive to this drug, just as they are sensitive to many 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. See information on screening.

Prazosin is best given with food.

It is our policy not to give dosing information over the Internet.