Megalis 20 mg contains tadalafil, a phosphodiesterase type 5 (PDE5) inhibitor used to treat erectile dysfunction (ED) in men. Tadalafil works by enhancing the effects of nitric oxide, a natural chemical the body produces to relax muscles in the penis. This increases blood flow, allowing for a firmer, longer-lasting erection during sexual arousal. Tadalafil, the active ingredient, has a long half-life of about 17.5 hours, allowing its effects to last up to 36 hours. This extended window means the user doesn't have to time the dose as precisely as with other ED drugs like sildenafil.