Megalis 20 mg belongs to a class of medications known as phosphodiesterase type 5 (PDE5) inhibitors. The active ingredient, Tadalafil, works by inhibiting the action of the enzyme PDE5, which can restrict blood flow to the penis. By blocking this enzyme, Tadalafil helps to relax the blood vessels, allowing more blood to flow into the penis during sexual arousal, thereby improving erectile function.