Brand name(s):  Minipress®

Available as: 1mg, 2mg, and 5mg capsules

May be available in compounded form if needed.

How this Medication is Used

The two most common uses for prazosin are to reduce high blood pressure and to relax the urinary sphincter and the urethra that delivers urine from the bladder to the outside.

Drugs can be used to change the activity of muscles in the body. The muscle cells have receptors that transfer signals from hormones, neurotransmitters, and other such natural chemicals in the body to the cell and influence what it does. For example, smooth muscle in arteries have alpha receptors that influence their contraction. Drugs called agonists (meaning like) bind to the specific receptor and cause the same effect as the natural signal (e.g., acetylcholine or epinephrine).

Alpha agonists cause smooth muscle in the arteries to contract, which can cause high blood pressure. Drugs acting as antagonists (meaning opponents) bind to the receptor and prevent the effect so that the cell cannot do what the natural chemical wants it to do. In the case of alpha antagonists, because the muscle is not told to contract, it stays relaxed.

Prazosin is an alpha-1 antagonist. This means that it prevents the effects of epinephrine on alpha-1 receptors found in smooth muscles. Smooth muscles are found throughout the body, but locations influenced by prazosin include blood vessels, the urinary bladder, and the gastrointestinal tract.

Urinary Conditions

Urination may become difficult if the sphincter and urethra leading from the bladder to the outside are narrowed. Relaxing the smooth muscle in these areas may help urine pass. Some of the conditions in which the narrowing occurs include idiopathic cystitis in cats, patients with bladder or prostate tumors, or patients with spinal disease.

Blocking or antagonizing alpha receptors may help relax the smooth muscle. Urine may pass more easily in these and other conditions. However, several recent studies in cats have shown that prazosin is not effective in helping urine flow in patients with idiopathic cystitis, and it may actually make urine passage more difficult. Veterinarians may choose not to use it in cats because of this effect. Prazosin may still be useful to relax the urinary sphincter and urethra in dogs with prostate or other disorders.

Cardiovascular Disease

Prazosin might be used to relax the arteries in cats or dogs with high blood pressure or, in some cases, congestive heart failure.

Dosing

Prazosin is available as a capsule approved for use in humans. The pioneer (first drug approved) is called Minipress. There are multiple generic products approved for use in humans.

Your veterinarian may prescribe Prazosin to be given once every eight, 12, or 24 hours. Following your veterinarian’s recommendations is important.

Prazosin can be given with food or without food. 

If a dose is missed, and the missed dose is caught before half of the dosing interval has passed, the missed dose can be given. For example, if the interval is eight hours, and the missed dose is caught before four hours have passed, the missed dose can be given. If the interval is 12 hours and the missed dose is caught before six hours have passed, the missed dose can be given.

Store prazosin at room temperature.

Side Effects

Lethargy (sleepiness) is the most common side effect in cats. The third eyelid may be raised in cats although this is only a sign of lethargy and not a problem to be corrected. It will go away once the effect of prazosin wears off.

At higher doses, weakness due to very low blood pressure and a rapid heart rate may occur in dogs or cats. These signs can indicate an overdose. Your veterinarian should be called at once if an overdose or side effect is suspected.

Gastrointestinal effects such as vomiting, diarrhea, constipation, or appetite loss can also occur. These effects might be reduced by giving the medication with a small amount of food.

Interactions with Other Drugs

Some patients require multiple drugs to control blood pressure. Other drugs may control blood pressure through different mechanisms. Using two drugs for the same effect may increase the risk of low blood pressure.

Examples of medications that might cause blood pressure to be too low in patients receiving prazosin include ACE inhibitors such as enalapril or benazepril, angiotensin receptor blockers such as telmisartan, calcium channel blockers such as amlodipine or diltiazem, or beta-blockers such as atenolol.

Other drugs that are less likely to increase the risk of low blood pressure include pentoxifylline, sildenafil, and pimobendan. It is important that your veterinarian be aware of any drug or supplement your pet is receiving.

Concerns and Cautions

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