It continues to work for up to 4 hours, although its action is usually less after 2 hours. This medicine comes with a patient information leaflet. This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. Click here for an email preview. Slowly pull back the plunger until the required dose. Ask your healthcare professional how you should dispose of any medicine you do not use. · Sildenafil Dosage Medically reviewed by Last updated on Applies to the following strengths: 10 mg/mL; 10 mg/ mL; 20 mg; 25 mg; 50 mg; mg Usual Adult Dose for: Erectile Dysfunction Pulmonary Hypertension Usual Geriatric Dose for: Erectile Dysfunction Additional dosage information: Renal Dose . The normal dose when starting Viagra is 50 mg, as needed. The maximum dose of Viagra is mg each day. Doses above the maximum dose, such as mg per day, will increase the risk of side effects.  · For oral dosage form (tablets).