It works for up to 36 hours. Check with your doctor immediately if any of the following side effects occur: Less common burning feeling in the chest or stomach burning, crawling, itching, numbness, prickling, “pins and needles”, or tingling feelings cloudy or bloody urine. Do not take other medicines unless they have been discussed with your doctor. When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction. If you often forget doses, it may help to set an alarm to remind you.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. Viagra comes as tablets that you take by mouth. It’s available in three strengths: 25 milligrams (mg), 50 mg, and mg. Dosage for ED. The typical dose of Viagra for erectile dysfunction (ED. · The effects may be increased because of slower removal of the medicine from the body. Leukemia (blood related cancer) or. Multiple myeloma (blood related cancer) or. Sickle-cell anemia (blood disorder)—Sildenafil should be used with caution in these patients as problems with prolonged erection of the penis may occur.