Cialis Tablet In Pakistan Cialis, Viagra, Levitra are the most popular effective impotence treatment drugs available online on prescription. These drugs improve the erection, regardless of whether the cause of the male impotence is physical or psychological. Viagra, Cialis, Levitra are all similar and they work in the same way. They are chemically referred to as PDE5 inhibitors. However the effects they give vary between the drugs. There are major differences between how long the drug works and how quickly it works. Viagra and Levitra for approximately 4 hours and they both take effect in 30 minutes. Cialis works a bit faster (within about 15 minutes), and the effects last much longer upto 36 hours in most cases.