What Is Tadalafil? Tadalafil is a phosphodiesterase type 5 (PDE5) inhibitor. It increases blood flow to specific areas of the body, particularly the penis, by relaxing blood vessel walls. Unlike other ED drugs with shorter duration, Tadalafil is often referred to as the “weekend pill” due to its prolonged effect window of 24–36 hours. It's available under various brand names,...