Recent heart or blood vessel problems. Sildenafil usually should not be taken more than once every 24 hours. Medicines that block an enzyme called CYP3A4. These include itraconazole Sporanox, Tolsura , ketoconazole Extina, Ketozole, others and ritonavir Norvir. If you are taking sildenafil to treat erectile dysfunction, follow your doctor’s directions and the guidelines in this paragraph. Proper Use Use sildenafil exactly as directed by your doctor. Dosing The dose of this medicine will be different for different patients. · Viagra is used as needed, meaning you only take a dose prior to sexual activity. Cialis can be prescribed for use as needed, but can also be prescribed for use once per day, every day (without. Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum recommended dose is mg. Don’t take sildenafil (Viagra) more than once daily (at any dose). Written by Brian Leonard, PharmD, BCACP, BCGP. The recommended dosage of Viagra for erectile dysfunction (ED) is 50 mg, taken as needed about 1 hour before sexual activity. You can take it 30 minutes to 4 hours before sexual activity. And you.