Though all three of these drugs belong to a class of drugs known as PDE-5 inhibitors, each is slightly different, and TADALAFIL 20mg is perhaps the most distinctive. Cialis has a much longer half-life than Sildenafil 100mg and Levitra (Vardenafil), and Eli Lilly, manufacturer of Cialis, has capitalized on this property for years. Because the effects of Cialis can last for up to 36 hours, the drug was given the nickname “the weekend pill,” since a man could theoretically take one on Friday night and still experience the effects on Sunday. This, the company said, could allow more spontaneity thanor Levitra, which wear off within a few hours.