What special dietary instructions should I follow? What special precautions should I follow? It works for up to 36 hours. For treatment of pulmonary arterial hypertension: For oral dosage forms suspension or tablets : Adults—20 milligrams mg 3 times a day. Take sildenafil at around the same times every day. What, if anything, seems to worsen your symptoms?Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. This boosts blood flow and helps you get an erection from sexual activity. How oral medicines differ. Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. This increased blood flow can cause an erection. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. Oral medications are a successful erectile dysfunction treatment for many men. They include: Sildenafil (Viagra) Tadalafil (Adcirca, Cialis) Vardenafil (Levitra, Staxyn) Avanafil (Stendra) All four medications enhance the effects of nitric oxide — a natural chemical your body produces that relaxes muscles in the penis. See more.