Compare the benefits and drawbacks of Sildenafil vs Cialis, including functions, effsildenafil (Viagra) usually works for about 4 to 6 hours and should be taken as needed, around an hour before sex. tadalafil (Cialis) can last up to 36 hours, and it can be taken as needed or daily in a low dose. This increases the risk of side effects, including serious ones like a dangerous drop in blood pressure. ectiveness and uses for erectile dysfunction treatment