Use only the brand of this medicine that your doctor prescribed. How should this medicine be used? Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine. Take sildenafil at around the same times every day. Do not stop taking sildenafil without talking to your doctor. Children 1 year of age and older weighing 20 to 45 kg—20 mg 3 times a day. It is important to tell your doctor about any heart problems you have now or may have had in the past.Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . Fake Viagra Pills Continue to Flood the Market Phony pills are rampant. Viagra, the first oral drug in the U.S. specifically prescribed for impotency, was approved by Viagra, . 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.