Kidney disease that needs to be treated with dialysis. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. Sildenafil usually should not be taken more than once every 24 hours. It works for up to 36 hours. Your doctor or other health care professional can recommend one for you based on these factors, other health conditions you have and any other medicines you take. Patients should always ask their doctors for medical advice about adverse events. Generic versions of sildenafil, vardenafil and tadalafil are available.Side Effects. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may. According to price-comparison site test1.ru, generic Revatio will cost you as little as $ per pill, depending on where you fill your prescription. That’s compared with . FAQs What to consider Cost Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. .