By Rania Gollakner, BS, DVM, MPH

What is sildenafil?

Sildenafil (brand names: Viagra, Revatio, Aphrodil, Vizarsin, Granpidam) is a vasodilator used to treat high blood pressure in the lungs. It is also used to treat megaesophagus. In cattle, it may be useful in treating uterine conditions such as endometritis. It is mainly used in dogs.

It is used in dogs and occasionally cats to treat high blood pressure in the lungs and is ‘off label’ or ‘extra-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 direction 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 setting. It should be administered on an empty stomach at least 1 hour before food or 2 hours after food. If vomiting occurs when dosed on an empty stomach, give future doses with food. If vomiting continues, contact your veterinary office. Measure liquid doses very carefully. Some liquid forms must be shaken well before dosing. Read the label for further instructions.

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?

You should give the missed dose as soon as you remember and then wait the recommended amount of time between doses. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

Side effects are uncommon but may include skin flushing in the groin region, and 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.

This short-acting medication should stop working within 24 hours, although 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 or that are currently taking nitrate medications. Use cautiously in pets with certain heart diseases such as congenital shunt, ischemic disease, obstructive hypertrophic disease, ventricular arrythmias, or left-sided heart disease. Use extremely cautiously in pets with low blood pressure, severe dehydration, or bleeding disorders and adjust doses in pets with liver or kidney disease.

Safety during pregnancy and in pets that are nursing has not been established. Sildenafil should be used cautiously and only when benefits outweigh the risks.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with sildenafil: ACE inhibitors, alpha-adrenergic antagonists, alpha-2 adrenergic agonists, amlodipine, angiotensin receptor blockers, azole antifungals, cimetidine, ciprofloxacin, clarithromycin, erythromycin, furosemide, heparin, nitrates, nitroprusside sodium, paclitaxel, phenobarbital, propranolol, or rifampin.

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?

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 side effects.

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. For compounded liquid formulations, storage instructions may differ. Follow the storage instructions on the label.

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.

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