Sildenafil

What is sildenafil?

Sildenafil (brand name: Viagra®) is a vasodilator, which means it helps relax and open the blood vessels.

It is used off label (extra label) to treat high blood pressure in the blood vessels of the lungs (pulmonary hypertension) in dogs and occasionally cats. It also may be used off label to treat megaesophagus in dogs and cats.

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 sildenafil given?

• Sildenafil is given by mouth in the form of a tablet or a liquid. It can also be given by injection in the clinic.
• Measure liquid doses carefully. Some liquid forms must be shaken well before dosing. Read the label for further instructions.
• Give sildenafil on an empty stomach at least one hour before food or two hours after food. 
• If vomiting occurs when dosed on an empty stomach, give future doses with food. If vomiting continues, contact your veterinary office. 

This medication will take effect quickly, in about 1 to 2 hours, and improvement in clinical signs should follow.

What if I miss giving my pet the medication?

If you miss a dose, give it as soon as you remember, and then wait the recommended time for the next dose, as prescribed by your veterinarian. Do not give the pet two doses at once.

Are there any potential side effects?

Sildenafil may cause the following side effects: 

• Skin flushing in the groin region
• Gastrointestinal upset such as decreased appetite, vomiting, and diarrhea

In humans, side effects may include headache, blindness, nosebleeds, deafness, dizziness, indigestion, muscle aches, and nose congestion. The significance of these effects in dogs and cats is unknown.

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 last longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT USE this medication in:

• Pets that are allergic to it 
• Pets that are currently taking nitrate medications

USE WITH EXTREME CAUTION in:

• Pets with low blood pressure
• Pets with severe dehydration 
• Pets with bleeding disorders
• Pets with liver or kidney disease (adjust dose)
• Pets that are pregnant or nursing (safety has not been established; use only when the benefits outweigh the risks)

USE WITH CAUTION in:

• Pets with certain heart diseases, such as congenital shunt, ischemic disease, obstructive hypertrophic disease, ventricular arrhythmias, or left-sided heart disease

Are there any drug interactions I should be aware of?

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

• ACE inhibitors
• Alpha-adrenergic antagonists
• Amlodipine
• Angiotensin receptor blockers
• Azole antifungals
• Furosemide
• Phenobarbital
• Propranolol

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

Is any monitoring needed with this medication?

• Blood pressure in the lungs and in the body should be monitored regularly. 
• Monitor your pet to be sure that the medication is working by noting improvements in ability to exercise, cough, breathing effort, or weight gain.
• 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 sildenafil?

• Store this medication at room temperature (around 77°F or 25°C). The liquid form may be stored at room temperature or in the refrigerator, but it should be protected from freezing. 
• Discard the liquid form 60 days after reconstitution. 
• 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 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.