Here is an article on this exact question:
Cialis can build up in the body
Someone using one Cialis tablet every two days will find Cialis levels go on rising to reach a steady state after about a week. On day 2, when taking a second tablet around 20% of the original tablet is still in the blood stream. On day 4, 22% of a single dose will still remain. On day 6 around 24% and so on. So someone who finds a single dose still has an effect after 36 hours could in theory find that a tablet every other day enables them to have sex whenever they wish without fear of failure - assuming reasonable intervals before doing so.
Of course, other factors are important such as the possibility of more continuous or more serious side effects - after a single dose 14% experience headaches and 9% heartburn.Taking Cialis in such a way should not be considered by anywone under any circumstances without close medical supervision and advice.
In France, it has been dubbed ‘Le Weekend’ pill. Unlike Viagra, Cialis does not interact with with food or alcohol - Viagra is best taken on an empty stomach. GlaxoSmithKline/Bayer’s Levitra is expected to launch in 2003. Pfizer is trying to block US launches of Cialis and Levitra, but similar legal action in Europe failed on the grounds that the information to be patented was already in the public domain.
Cialis - impotence pill - side effects