It continues to work for up to 4 hours, although its action is usually less after 2 hours. Tell your doctor if any of these symptoms are severe or do not go away: headache. Its effects often last for 4 to 5 hours. Continue to take sildenafil even if you feel well. Bleeding disorders, history of or Stomach ulcer, or history of or—Chance of problems occurring may be increased. Ask your healthcare professional how you should dispose of any medicine you do not use. If you have an erection that lasts more than 4 hours, get medical help right away.Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash.  · 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 penis is one of the areas where . Viagra is effective because it slows the action of phosphodiesterase type 5 (PDE5), an enzyme in the blood vessel walls. This helps the muscles in the penis to relax and encourage blood flow.