By Krista Williams, BSc, DVM, CCRP

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 as a treatment for congestive heart failure, systemic hypertension, or pulmonary hypertension in dogs.

Its use in pets is ‘off-label’. Many drugs are commonly prescribed for off-label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their directions 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. It may be given with or without food; however, if vomiting occurs when dosed on an empty stomach, give future doses with food. Measure liquid forms carefully.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

Common side effects include lethargy, vomiting, diarrhea, nausea, and constipation. Serious side effects include fainting, excessive tiredness, weakness, or dizziness. Contact your veterinarian if you observe any of these signs in your pet.

Are there any risk factors for this medication?

Do not use prazosin in pets that are allergic to it. It should be used with caution in pets with kidney disease or in animals with conditions that cause low blood pressure.

Some breeds of dogs (e.g., collies, sheepdogs, and collie- or sheepdog-cross breeds) have a specific genetic mutation (MDR1) that makes them less able to tolerate certain medications. These dogs are believed to be more susceptible to the hypotensive effects of prazosin. This medication should be used in MDR1-positive dogs with extreme caution and, if administered, lower doses and increased monitoring are recommended.

Your veterinarian will advise you on the safety of prazosin use in your pet.

Are there any drug interactions I should be aware of?

There are many medications that should be used with caution when given with prazosin including ACE inhibitors such as benazepril (Fortekor®) and enalapril (Enacard®), amlodipine, beta-blocking agents such as propranolol (Inderal®), clonidine (Kapvay®), pentoxifylline, sildenafil (Viagra®), and telmisartan (Semintra®).

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

Is there any monitoring that needs to be done 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. Monitor your pet at home for serious side effects.

How do I store prazosin?

Store the capsules at room temperature, ideally between 68-77°F or 20-25°C, and protect them from moisture and light. For compounded formulations, follow the storage instructions on the label.

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.

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