You can take it in a small dose daily or in a larger dose as needed. It works for up to 36 hours. How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. Generic versions of sildenafil, vardenafil and tadalafil are available. Box , Mission, KS If your pharmacy does not participate, you may be able to submit a request for a rebate in connection with this Savings Offer.The average length of time Viagra lasts. On average, Viagra lasts between 2 and 3 hours before its effects start to diminish. Viagra can last up to 5 hours or longer depending on . What does Viagra do for ED? Viagra works to treat ED by helping you have and maintain an erection. The drug does this by improving blood flow into your penis when . It’s simple: When it works as intended, Viagra causes a man who is sexually stimulated to get an erection. How does Viagra do that? And why does Viagra work only if the .