Often, these are prescribed for chest pain called angina. It works for up to 36 hours. Take sildenafil exactly as directed. Avanafil Stendra. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St.Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Generic Viagra (sildenafil) is an alternative to the branded Viagra drug that can be used for treating erectile dysfunction (ED). It’s now sold by many manufacturers, including online. You may take it up to 4 hours before sexual activity. Do not take Viagra more than once per day. Shake the oral suspension (liquid) before you measure a dose. Use the dosing .