If you are taking sildenafil for PAH, take the missed dose as soon as you remember it. Its effects often last for 4 to 5 hours. This will increase the supply of blood to the lungs and reduce the workload of the heart. Generic versions of sildenafil, vardenafil and tadalafil are available. Sildenafil is also used in both men and women to treat the symptoms of pulmonary arterial hypertension.Viagra is an effective drug for treating male erectile dysfunction, but it targets sexual performance, not sexual arousal. There is limited evidence that Viagra may be beneficial for women with sexual dysfunction, but the . In most males,* Viagra works within 1 hour after it’s taken. It’s possible that Viagra can begin working within 30 minutes of being taken. But other times, it may take up to 4 hours to start.  · Viagra; Descriptions. Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase 5 (PDE5) inhibitors. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly. The .