And what about drugs that decrease the effect of Viagra on your erections? Its effects often last for 4 to 5 hours. They include dutasteride Avodart, Jalyn and finasteride Propecia, Proscar. It works for up to 36 hours. Tadalafil Cialis. Common ones include: Flushing of the skin.Do not take Viagra while also taking Revatio, unless your doctor tells you to. Warnings Some medicines can cause unwanted or dangerous effects when used with Viagra. Don’t be fooled into buying “herbal viagra.” Never take any medicines that claim to be the “herbal” or nonprescription version of an oral medicine for ED. These aren’t an . Do not use more Viagra than your doctor prescribes. Using more than this can lead to serious side effects. What to do in case you take too much Viagra.