You can take sildenafil with or without food. John’s wort. Do not stop taking sildenafil without talking to your doctor. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Ask your doctor if you have any questions. Tell your doctor if sildenafil is not working well or if you are experiencing side effects. For example, your doctor might ask you not to eat before having a blood test.Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). In combination with sexual stimulation, sildenafil works by increasing blood flow to the penis to.  · Sildenafil (Oral Route) Description and Brand Names. Before Using. Proper Use. Precautions. Side Effects. Products and services. Proper Use. Drug information provided by: Merative, Micromedex ®. Use sildenafil exactly as directed by your doctor. Do not use more of it and do not use it more often than your doctor ordered. Viagra is a brand-name prescription drug that’s approved to treat erectile dysfunction (ED).With ED, you’re unable to have or maintain an erection. Viagra is approved for this use in males.