So what are the differences when you buy Generic Viagra online and Cialis? Although each works in a similar way, there are some characteristics that differentiate them: side effects, how long they work and when they should be taken. For example, although it takes 30 to 60 minutes for both Viagra and buy Generic Cialis in Canada to be absorbed into the bloodstream, studies show that Cialis can work for up to 36 hours, while Viagra only works for four to six hours. In addition, the effectiveness of Cialis is generally unaffected by food, while Viagra may be more effective when taken on an empty stomach. Another key point worth mentioning is that Cialis is also used to treat benign prostatic hyperplasia. This is a condition in which the prostate enlarges. If left untreated, it can block the flow of urine out of the bladder and possibly cause urinary tract problems or kidney disease.