It has long been suspected that surgery to remove cancers can cause them to spread. It is very possible that the drugs used during surgery can cause cancer cells to grow or activate dormant cancer cells.
It turns out that when ketorolac, a pain killer otherwise known as toradol, was given prior to starting mastectomy surgery for breast cancer, the rate of recurrence reduced from 17% to 6% over 2 years. To see this difference, patients had to get the drug before surgery started. That is an enormous difference. The patients in this study did not receive narcotics like morphine or codeine -- those are known to suppress the immune system.
Drugs that are used in surgery affect the immune system and it turns out that a particular type of cell called the "Natural Killer" cell is inhibited by surgery. These cells are the first line of defense against cancers and help guard against their spread. Cutting down on narcotics and administering ketorolac may give these cells the ammunition they need to kill cancer.