This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. Click here for an email preview. But medication may be just one of many ways to help you stay hard. To use the oral liquid: Shake the bottle well for at least 10 seconds before each use. Place the tip of the syringe into you or your child’s mouth and point it towards the cheek. Within 24 hours, a healthcare provider will contact you for further information and to present you with several options. Do not keep outdated medicine or medicine no longer needed.For treatment of erectile dysfunction: For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before sexual . 50 mg mg Typical dosages The following information describes commonly recommended dosages for ED. However, be sure to take the dosage your doctor . You should not use Viagra if you are allergic to sildenafil, or: if you take other medicines to treat pulmonary arterial hypertension, such as riociguat (Adempas). if you take nitrates. .