As for me, I used to get pretty bad side effects (headache, swollen nose, distorted peripheral vision) from Viagra and my doc switched me to Cialis. Cialis is also known as the “Weekend Pill” because of its long-lasting effects which can last up to 36 hours. In comparison, Viagra only endures for up to 5 hours even though it contains a stronger dosage of an active agent than Cialis. You can find more info on this Canadian pharmacy site)