06/22/2026
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Yes, we’re talking about sildenafil.
No, not for the reason you’re thinking 👀
Sildenafil is a phosphodiesterase-5 (PDE5) inhibitor that acts as a vasodilator, meaning it relaxes the smooth muscle within blood vessels. In horses, this primarily affects the pulmonary circulation, allowing blood vessels within the lungs to dilate and reducing pressure within the pulmonary arteries.
By decreasing pulmonary vascular resistance, sildenafil can reduce the workload placed on the right side of the heart and improve blood flow through the lungs.
Veterinarians may consider sildenafil in select respiratory and cardiovascular cases, including:
Pulmonary hypertension
Severe equine asthma (formerly recurrent airway obstruction, RAO)
Cases of exercise-induced pulmonary hemorrhage (EIPH) in conjunction with other therapies
Certain forms of right-sided cardiac disease
Equine asthma is characterized by airway inflammation, mucus accumulation, and bronchoconstriction that commonly develop secondary to environmental triggers such as dust, mold, or poor air quality. Chronic inflammation within the lungs can increase resistance within the pulmonary circulation, resulting in pulmonary hypertension.
Research has demonstrated that horses with severe equine asthma may develop elevated pulmonary arterial pressures, and pulmonary hypertension has been proposed as one contributing factor in some cases of exercise-induced pulmonary hemorrhage (EIPH).
Because sildenafil lowers pulmonary blood pressure, it has been investigated as an additional therapy in difficult respiratory cases and in some horses that continue to experience bleeding episodes despite conventional management.
Sildenafil is sometimes used alongside furosemide (Lasix), particularly in complicated EIPH cases. However, both medications can influence blood pressure and fluid balance. When used together, careful veterinary supervision is important, as excessive reductions in blood pressure may occur in some horses.
It is important to remember that sildenafil does not treat the underlying cause of respiratory disease. Environmental management, dust reduction, forage quality, weight management, and appropriate treatment of airway inflammation remain essential components of therapy.
Like many medications used in equine medicine, sildenafil is a tool, not a cure. The goal is to improve pulmonary function and quality of life while addressing the underlying disease process whenever possible.
