If you are taking sildenafil to treat erectile dysfunction, follow your doctor’s directions and the guidelines in this paragraph. Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. The studies that established benefit demonstrated improvements in success rates for sexual intercourse compared with placebo. Both Cialis and Viagra are effective at these doses. Males between the ages of 30 and 65 would be eligible to buy four tablets after a consultation with a pharmacist. People who use nitrates in any form or are being treated for pulmonary hypertension should not take sildenafil. In addition, ED is extremely common.Dosage. The dose of Cialis you need will depend on whether you’re taking it daily or as needed. Daily doses are often between and 5 milligrams (mg). The as-needed dose is usually 10 mg but can range from 5–20 mg. The typicaldose of Viagra is 50 mg. However, the dosage can range from 25– mg. Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work. Viagra comes as tablets that are taken by mouth. In most cases, it should be taken about an hour before sexual activity, but it can be taken 30 minutes to 4 hours beforehand.