The efficacy of Viagra and Cialis varies from person to person, which means that some people may do better with Viagra and some people may do better with Cialis. The scientific name of Viagra is Sildenafil and the scientific name of Cialis is Tadalafil. Both drugs are used to treat erectile dysfunction. Viagra has a very short half-life and will generally require that it be taken 30 minutes before sexual activity. In many cases there is no way to determine when sexual activity is going to take place, so taking it 1-4 hours before sexual activity is fine. Cialis is a long half-life drug and can be very effective when taken at any time of the day. The fact that the duration of administration is not affected by the anticipated sexual activity is the biggest advantage of Cialis. In order for either Viagra or cialis to be very effective, there must be sexual stimulation or arousal. If there is no stimulation, neither drug can assist in causing an erection, which means there is no way to treat erectile dysfunction.