Prazosin

What is prazosin?

Prazosin is an alpha-adrenergic blocker used to reduce urinary sphincter tone to relax the bladder and allow easier urination in dogs and cats. It may also be used to treat congestive heart failure, systemic hypertension, or pulmonary hypertension in dogs.

Its use in pets is off label (extra label). Many drugs are commonly prescribed for off-label use in veterinary medicine. “Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they may be significantly different from those on the label.

How is prazosin given?

• Prazosin is given by mouth in the form of a capsule. It may also be specially compounded into other formulations.
• Measure liquid forms carefully.
• It may be given with or without food; however, if vomiting occurs when dosed on an empty stomach, give future doses with food.

What if I miss giving my pet the medication?

If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. If it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects?

Prazosin may cause the following side effects:

• Lethargy
• Nausea
• Vomiting
• Diarrhea
• Constipation

Serious adverse reactions may occur. If any of the following signs develop, stop giving prazosin and contact your veterinarian immediately:

• Fainting
• Excessive tiredness
• Weakness
• Dizziness

Drug sensitivities can develop over time, with repeated exposure. Your pet may have no reaction after the first few doses but may develop a sensitivity later. It is important to watch for signs of an allergic reaction over the entire course of treatment (e.g., irregular breathing, rash, fever, puffiness and swelling around the face) and notify your veterinarian if you have any concerns. 

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately. This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT USE prazosin in:

• Pets that are allergic to it

USE WITH CAUTION in:

• Pets with kidney disease
• Pets with conditions that cause low blood pressure

USE WITH EXTREME CAUTION in:

• MDR1-positive dogs

Some dog breeds are more sensitive than others to medications (e.g., collies, sheepdogs, and collie- or sheepdog-cross breeds). This sensitivity is typically due to a genetic mutation (MDR1/ABCB1) that makes them less able to tolerate high doses of certain medications. Use prazosin with extreme caution in MDR1-positive dogs. Lower doses and increased monitoring are recommended, if it is administered. Your veterinarian will advise you on the safety of prazosin use in your pet.

Are there any drug interactions I should be aware of?

Several medications should be used with caution when given with prazosin, including:

• ACE inhibitors such as benazepril and enalapril
• Amlodipine
• Beta-blocking agents such as propranolol, pentoxifylline, sildenafil, and telmisartan

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any treatment.

Is any monitoring needed with this medication?

• Your veterinarian may monitor your pet to be sure that the medication is working, including monitoring for adequate urination or for blood pressure. Your veterinarian will let you know how frequently this monitoring needs to be performed.
• At home, monitor for any side effects such as those noted above. If any occur, stop using the medication and contact your veterinarian.

How do I store prazosin?

• Store the capsules at room temperature, ideally between 68°F and77°F (20°C and 25°C), and protect them from moisture and light.
• Store compounded forms of this medication according to the label.
• Keep the container out of reach of children and pets.

What should I do in case of an emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet's weight and symptoms.