The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Children 1 year of age and older weighing 45 kilograms kg or more—20 mg 3 times a day. You may opt-out of email communications at any time by clicking on the unsubscribe link in the e-mail. This medicine comes with a patient information leaflet. Sildenafil Viagra. Dosing The dose of this medicine will be different for different patients.Sildenafil oral tablet is available as the brand-name drugs Viagra and Revatio. It’s also available as a generic drug. Generic drugs usually cost less than the brand-name . you should know that sildenafil is available under the brand names Liqrev, Revatio, and Viagra. You should only be treated with one of these products at a time. The following . Viagra comes as an oral tablet that contains the active drug sildenafil. Sildenafil is also available in generic form. Viagra is classified as a phosphodiesterase type 5 (PDE5) .