Viagra is the trade name for Sildenafil and Cialis is the trade name for Tadalafil, and there is no clear conclusion as to who is more harmful between these two drugs. Common side effects of sildenafil include headache, facial flushing, indigestion, and muscle pain. Also rare are nasal congestion, diarrhea, and dizziness. In addition to these side effects, transient vision abnormalities, manifested as impaired color discrimination or increased sensitivity to light, are occasionally seen and the risk is increased with high doses. Some people who use tadalafil experience side effects such as dry mouth, back muscle pain, muscle weakness, and mild flushing of the face, and an overdose can cause frequent headaches. These two drugs are in the same class and work on similar principles. The adverse effects are close to similar. Their main difference is the different half-life of the drug. Sildenafil prohibits the use of drugs in combination with nitrates, the drug is contraindicated in women, people who are allergic to the product, children, people whose recent cardiovascular state does not allow them to have sex. Tadalafil is contraindicated in people who are allergic to any of the components of tadalafil, who are taking any form of nitrates, guanylate cyclase stimulators, women, children and other people. Both drugs are prescription medications and should be applied in strict compliance with the doctor’s instructions.