You can take it up to 4 hours before you want to have sex. Side effects Side effects of oral ED medicines are often mild. These medicines are called PDE5 inhibitors. Sildenafil at steady state, at a dose not approved for the treatment of erectile dysfunction 80 mg t. You can take sildenafil with or without food. Some medications should not be taken with sildenafil.Safety & Possible Side Effects. take any medicines called nitrates, often prescribed for chest pain, or guanylate cyclase stimulators like Adempas (riociguat) for pulmonary hypertension. Your blood pressure could drop to an unsafe level. are allergic to sildenafil, as contained in VIAGRA and REVATIO, or any of the ingredients in VIAGRA.  · If you have questions about Viagra dosages based on your age, talk with your doctor or pharmacist. Is Viagra prescribed in dosages of mg, mg, or mg? About sildenafil (Viagra) Sildenafil is a medicine used to treat erection problems (erectile dysfunction). Sildenafil is also sometimes used to treat pulmonary hypertension (high blood pressure in the blood vessels that supply the lungs). If you have erection problems sildenafil works by temporarily increasing blood flow to your penis when.