Viagara: Has an active life of about 4 hours, but can last up to 5 hours post-consumption, due to digestive lag time....with slight effects remainng for 6-7 hours. This drug hits like a locomotive, leaving no question as to whether or not it is "working". Viagra is the most effective ED drug for the largest number of people and begins to take effect only 30-60 minutes after consumption. If you are able to plan sex within a 4 hour window, you will likely have the best results with this drug. 50-100 mg is the standard dose, although for many, 100 mg is excessive and unecessary. Along with its typical greater potency, it also has more pronouced side effects, such as blurred vision, stuffy nose, pressure behind the eyes, and possible headache (similar to the type of headache one would get with sinus problems).
Cialis: Has an active life of about 20 hours. This drug generally has less intense effects than Viagra, but is more effectively than Levitra. The drug takes about 4-5 hours to reach maximum effectiveness, but may be felt mildly in 2-3 hours. Side effects are less pronounced than with Viagra. This drug is more suited to those who cannot plan sex as accurately, due to its greater window of effectiveness.
Levitra: Sometimes known as the weekend warrior, Levitra has a active life of about 40 hours. Generally, it is the least potent of the 3 ED drugs and for some people, it is not effective. It is not anywhere as potent, in terms of effectiveness, as Viagra. The upside, for those who do experience acceptably positive effects, is its extended duration of action. If you are hoping to "get lucky" at some point over the weekend and don't want to be caught off gaurd at a chance encounter, this drug can be takan well in advance without any fear of missing its window of opportunity. However, it should be made clear that in the large majority of cases, this drug provides a relatively mild to at best, moderate improvement in sexual functioning.