Take sildenafil up to 4 hours before you want to have sex. Store mixed oral liquid in the refrigerator. It continues to work for up to 4 hours, although its action is usually less after 2 hours. If you forget to take it If you have pulmonary hypertension and you forget to take a dose of sildenafil, take it as soon as you remember unless it’s nearly time for your next dose. The effects may be increased because of slower removal of the medicine from the body.Like Viagra, certain forms of sildenafil are approved to treat ED in males* ages 18 years and older. For example, sildenafil mg, mg, and mg tablets are approved for this use.  · Sildenafil is used to treat erectile dysfunction and pulmonary arterial hypertension. Includes sildenafil side effects, interactions and indications. Sildenafil is used to treat erectile dysfunction (ED) and to reduce blood pressure in pulmonary arterial hypertension (PAH). It is taken when needed for ED and daily for PAH. A headache, stomach upsets, and flushing are the main side effects, and it should not be taken within 24 hours of a nitrate. 5. Tips.