The following information includes only the average doses of this medicine. Never take an extra dose to make up for a forgotten one. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive. If you do not have a plastic syringe or spoon, ask your pharmacist for one. May make this condition worse. Sildenafil should not be used with any other medicine or device that causes erections.Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . This medicine is available on prescription and you can also buy it from most pharmacies. It comes as standard tablets, chewable tablets, and as a liquid that you swallow. Key facts. It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex. 1. How it works Sildenafil may be used to treat erectile dysfunction (ED or impotence) in men (Viagra brand) or pulmonary arterial hypertension in both men and women (Revatio .