They are PDE5 inhibitors that allow for increased blood flow to the penis, leading to more frequent and harder erections. Cialis lasts longer. Viagra is always taken as needed, usually an hour before having sex. Tadalafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow more easily. Is 20mg of Cialis better than mg of Viagra?Side Effects. Headache, stomach upset, back pain, muscle pain, stuffy nose, flushing, or dizziness may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. To. 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. Here are some details about tadalafil: Learn more about the different dosages for Cialis here. Pros and cons at a glance Below are some advantages and .