If you experience a prolonged or painful erection for 4 hours or more, contact your doctor immediately. However the dose is usually not more than 40 mg 3 times a day. This medication is taken with or without food an hour before sex. It is not known if the medicine is safe for use in these patients. Tadalafil Cialis. If you are taking sildenafil to treat erectile dysfunction, follow your doctor’s directions and the guidelines in this paragraph. Trimazosin Other Interactions Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur.Priapism warning: This drug can cause priapism, which is an erection that won’t go away. If you have an erection that lasts more than 4 hours, call or go to the nearest emergency room. If it. Generic Names: Sildenafil Citrate Brand Names: Revatio Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). Learn more about . This means that 4 hours after you take a dose, your body has cleared half of the dose. In most cases, it takes about 5 half-lives for your body to completely remove a drug from your system. So.