Remove the syringe from the bottle. Your doctor may adjust your dose as needed and tolerated. Do not use more of it and do not use it more often than your doctor ordered. Brand names Why is this medication prescribed? May make this condition worse. Follow the manufacturer’s directions to use and clean the oral syringe. To use the oral liquid: Shake the bottle well for at least 10 seconds before each use.What to consider Cost Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription.  · Sildenafil (Oral Route) Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase 5 (PDE5) inhibitors. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide.