Megalis 20 mg contains tadalafil, a phosphodiesterase type 5 (PDE5) inhibitor used to treat erectile dysfunction (ED) in adult men. Tadalafil increases blood flow to the penis during sexual stimulation, helping some men achieve and maintain an erection. It does not increase sexual desire and does not normally cause an erection without sexual stimulation. For ED, 20 mg is generally used as an as-needed dose, rather than a daily dose. The appropriate strength depends on medical history, kidney and liver function, other medicines, and individual response. Do not take more than prescribed or combine Megalis with other tadalafil-containing products or ED medicines without medical advice.