What side effects can this medication cause? You could also ask your pharmacist for advice on other ways to help you remember to take your medicine. Take sildenafil up to 4 hours before you want to have sex. Most patients receive a prescription for 50 mg of oral sildenafil, taken as needed approximately 1 hour before sexual activity. Benjamin P. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication.*for free flu shot & for $5 off $20 coupon: Flu vaccine is no cost with most insurance plans. Available at most CVS Pharmacy ® and MinuteClinic ® locations when a certified immunizer is on duty. Restrictions apply. Viagra (sildenafil) is a brand-name prescription drug that’s used to treat erectile dysfunction (ED). Learn about side effects, warnings, dosage, and more. Sildenafil is a medication used in the management and treatment of erectile dysfunction and pulmonary arterial hypertension. It is in the phosphodiesterase-5 inhibitor (PDE5-I) class of medications. This activity describes the indications, actions, adverse effects, and contraindications of sildenafil as an agent in the clinical setting. Knowledge .