Rarely reported side effects include: an erection that will not go away priapism. If you have an erection that lasts for more than 4 hours, you should go to the nearest hospital emergency department. Want to discuss your treatment options? Prolonged erections can be dangerous. It lasts 4 to 5 hours, or longer if you have mild to moderate ED.Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash. Women who use HSDD medications shouldn’t drink alcohol from 2 hours before they take the drug until the following morning, because it can lower blood pressure to dangerous levels. The drugs can. Drug Images What is Viagra? Viagra is a brand-name prescription drug that’s approved to treat erectile dysfunction (ED). With ED, you’re unable to have or maintain an erection. .