Before taking sildenafil, tell your doctor and pharmacist if you are allergic to sildenafil, any other medications, or any of the ingredients in sildenafil tablet or suspension. This site is intended only for U. How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup. However, if you take sildenafil with a high-fat meal, it will take longer for the medication to start to work. The decision, Teva Canada Ltd. Medicines that you take by mouth are called oral medicines.If you have these symptoms, stop taking VIAGRA and contact a doctor right away. have or have had heart problems such as a heart attack, irregular heartbeat, angina, chest pain, narrowing of the aortic valve, or heart failure. have low blood pressure, or high blood pressure that is not controlled. Erectile dysfunction is when a man has difficulty getting an erection or keeping it long enough for sex. It’s also known as ED or impotence. It happens when not enough blood flows to the penis, preventing an erection. The frequency increases with age, affecting about 50% of men over age 40*; however younger men may also experience ED. Viagra is effective because it slows the action of phosphodiesterase type 5 (PDE5), an enzyme in the blood vessel walls. This helps the muscles in the penis to relax and encourage blood flow.