Tadalafil is the generic version of Cialis, which lasts longer than other erectile dysfunction treatments (can last up to 36 hours). Tadalafil is a PDE5 inhibitor and is unique from the other PDE5 inhibitors because it's effects are long-lasting and can last for up to 36 hours. At this higher dose, Tadalafil should be taken when required. If you require two or more doses weekly, then you should consider the daily dose.