Sildenafil (Viagra) can be prescribed if you have been diagnosed with erectile dysfunction and are unable to get erections hard enough to have sex and complete sexual intercourse. Sildenafil is relatively safe and usually well tolerated with just a few mild side effects possible - changes in color vision, numbness, burning, or tingling, insomnia, diarrhea, light sensitivity, muscle aches, flushing, headache, heartburn, and nosebleeds,. You will need to notify you health care provider of any serious side effect you develop, or in case the mild side effects you got suddenly changed in intensity and became severe. Your doctor will need to know if you have any medical conditions that may require a dose adjustment for careful monitoring of your condition. You will need to report a history of heart attack, bleeding disorder, high cholesterol, conditions affecting the shape of the penis, pulmonary veno-occlusive disease, irregular heartbeat, stomach ulcer, chest pain, blood circulation problems, blood cell problems, diabetes, high or low blood pressure, heart, kidney, or liver disease, as well as a history of stroke.