What does Viagra do to a man? VIAGRA works for men with erectile dysfunction (ED) by increasing blood flow to the penis so you can get and keep an erection hard enough for sex. ... VIAGRA works only when you are sexually stimulated. You may take it up to 4 hours before sexual activity. How long does it take for viagra to work? 30 minutes How long does Viagra take to work? The effects of Viagra will vary from person to person. As a general rule, Viagra starts working in most people around 30 minutes after it's taken. Remember that taking Viagra alone will not cause an erection. BUY A COPY NOW.