Erectile dysfunction , or male impotence, is caused when blood flow to the penis is limiting the blood vessels, preventing a man from getting or keeping an erection hard enough for sex or any kind of sexual activity. It is completely safe to take Viagra or Sildenafil provided you are male and over, and have been prescribed it for erectile dysfunction. These drugs increase blood flow to the penis following sexual arousal, helping to produce an erection. Reduce your risk: As previously mentioned, some of the conditions that cause erectile dysfunction are preventable and reversible. References, Studies and Sources:. ED Tips How long does Viagra take to start working? You may take it up to 4 hours before sexual activity.VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are . Find the best sildenafil citrate or viagra prices near you now with our cost comparison tool and start saving today. QTY 30 • MG • Tablet • Near EDIT; Add to Medicine . Viagra comes as tablets that you take by mouth, when needed. Available strengths: 25 mg, 50 mg, mg. Viagra comes in these strengths: 25 milligram (mg), .