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Sildenafil for Veterinary Use
by Barbara Forney, VMD
Sildenafil is a selective vasodilator which works by inhibiting the enzyme phosphodiesterase V. This enzyme is found in relatively high concentrations in the lung and erectile penile tissue. Sildenafil preferentially relaxes the smooth muscle cells in the pulmonary vasculature, the corpus cavernosum, and other erectile tissue.
Sildenafil is approved for use in human medicine under the trade name of Viagra ®
. It is used for the treatment of pulmonary hypertension and erectile dysfunction in men. Its primary use in veterinary medicine is for the treatment of pulmonary hypertension in dogs and cats. It has been reported as a possible adjunctive treatment for exercise induced pulmonary hemorrhage (EIPH) in the horse.
Dogs and Cats
Sildenafil is used to treat moderate to severe pulmonary hypertension in dogs and cats. There are a number of causes of pulmonary hypertension, some of which originate with heart failure or cardiac abnormalities and some of which originate with pulmonary abnormalities. The mechanism of action of sildenafil occurs through vasodilation via smooth muscle relaxation of the pulmonary arterial vasculature. The effect on the pulmonary vasculature is independent of the underlying cause of the hypertension. Although there are other PEDVI drugs available on the human market, sildenafil has been the most studied in veterinary medicine. Improvement of clinical signs including exercise tolerance and quality of life is the most reliable indicator of therapeutic success. Improvement in arterial oxygenation has been demonstrated in humans and research in humans also shows that maximum pulmonary vasorelaxation occurs 60 minutes after oral administration of sildenafil.
Sildenafil has been used in the horse in an attempt to manage exercise induced pulmonary hemorrhage (EIPH). Its efficacy for this purpose has not been demonstrated. Sildenafil is used with oxygen and other supportive measures to decrease pulmonary vascular resistance for neonatal foals with delayed fetal/neonatal cardiopulmonary transition.
There has been some interest in the use of sildenafil for stallions with diminished libido. In most instances, this is not as effective as the stallion manager might hope. Erectile dysfunction in humans is usually a circulatory problem, not a libido problem. Loss of libido in stallions can be caused by a multitude of factors, but it is generally not caused by circulatory issues. Sildenafil will cause vasodilation and facilitate erection in the stallion, but it may not change or improve libido.
- Gastrointestinal upset, cutaneous flushing.
- Side effects in humans include headache, nasal congestion, difficulty breathing, prolonged erection, and dizziness.
- Sildenafil should not be used in animals with pulmonary veno-occlusive disease.
- Sildenafil should not be used in animals with hypotension, fluid depletion or shock, occlusion of left ventricular outflow, or dysfunction of the autonomic nervous system.
- Sildenafil should not be used with drugs that may cause hypotension such as nitrates, nitroprusside sodium, alpha-adrenergic blockers (phentolamine, phenoxybenzamine, phenothiazines), amlodipine, and any hypertensive drugs.
- Cimetidine, erythromycin, clarithromycin, ketaconazole, and itraconazole may reduce the metabolism of sildenafil.
- Phenobarbital and rifampin may decrease the concentrations of sildenafil.
- No information was found regarding overdose in animals. In a report concerning one person, clinical signs following a 20x overdose included cardiac rhythm changes, headache, dizziness, and flushing of the skin.
About the Author
Dr. Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania. She has a master's degree in animal science from the University of Delaware and graduated from the University of Pennsylvania School of Veterinary Medicine in 1982.
She began to develop her interest in client education and medical writing in 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
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