These medicines all work well for ED. This product is available in the following dosage forms: Tablet Suspension There is a problem with information submitted for this request. The effects may be increased because of slower removal of the medicine from the body. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. The most common side effects are headaches, feeling sick, indigestion and dizziness. However the dose is usually not more than 40 mg 3 times a day. Tadalafil Cialis.Viagra. 50 mg orally 1 hour before sexual activity. may be increased to mg or reduced to 25 mg, depending on effectiveness and tolerance. not to exceed mg/day. Pulmonary Arterial Hypertension. Revatio. Oral: 5 mg or 20 mg 3 .  · 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 four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). 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.