Viagra, also known as sildenafil, and tadalafil are similar drugs, both of which can be used to treat erectile dysfunction in men, and both are taken one hour before sexual activity. The duration of action of sildenafil is 4 hours, while the duration of action of tadalafil is longer, up to 36 hours. They also have similar side effects, with sildenafil being safer compared to tadalafil, which must be used on an empty stomach. This is because a high-fat diet and alcohol can delay absorption, whereas food does not interfere with the absorption of tadalafil. In a comparative trial of sildenafil and tadalafil, 66.3% of patients preferred tadalafil to 33.7% of patients for the treatment of erectile dysfunction. On average, intercourse occurred 2.2 hours after dosing in sildenafil-treated patients, whereas tadalafil-treated patients obtained sustained effects but were able to have intercourse 5.5 hours after the dosing delay. Any combination with any nitrate drugs such as isosorbide nitrate, isosorbide mononitrate, and nitroglycerin should be contraindicated at the time of use. The use of sildenafil should be delayed for 24 hours before the use of nitrates if sildenafil is used, and for 48 hours before the use of nitrates if tadalafil is used. Occasional blue vision may occur after sildenafil use, whereas tadalafil does not have blue vision.