Is the generic viagra real? Sildenafil is the generic for two common brand drugs on the market right now, Revatio and Viagra, but these drugs are approved by the FDA for two different uses. ... Revatio-equivalent sildenafil is only sold as a 20 mg tablet, while Viagra's generic sildenafil is sold in 25 mg, 50 mg, and 100 mg tablets. 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.