While all three medications are PDE-5 inhibitors and work in a similar way, they have differences in terms of how quickly they work and how long their effects last. Sildenafil (commonly known by the brand name Viagra) typically starts to work within 30-60 minutes and its effects can last up to 4-5 hours. Vardenafil (Levitra) also starts to work within 30-60 minutes and its effects last up to 4-5 hours. Tadalafil (Cialis), on the other hand, can start to work in as quickly as 15-20 minutes and its effects can last up to 36 hours.