Cancer patients have an increased risk of developing blood clots in the veins compared to non-cancer patients. Cancer patients who develop blood clots can lead to reduced life expectancy, delayed cancer treatment, and decreased quality of life. Prevention is the most effective way to decrease the complications associated with blood clots in the veins. Although previous clinical trials have shown some benefit on the use of medication to prevent blood clots in the veins in ambulatory cancer patients, these studies have been inconclusive in demonstrating that existing blood thinners significantly reduce the rate of blood clots in cancer patients. One possible explanation relates to the fact that these studies have included a large proportion of cancer patients who are a low risk of developing blood clots in the veins. We are proposing to identify cancer patients who are at a high risk of developing blood clots by using a validated tool at the time of their cancer diagnosis. The identified high risk cancer patients will be asked to participate in a trial to test the safety and efficacy of a new oral medication that has been used to prevent blood clots in patients undergoing surgery. We are enrolling 574 patients in 7 Canadian centers (Ottawa, Halifax, Montreal, Vancouver, Sault Ste. Marie, Toronto and Hamilton). 287 patients will receive the study drug and 287 will receive an inactive substance. Analysis will be performed to assess the safety and the superiority of the study drug.