Finally, after nearly two decades, Exelixis (Nasdaq: EXEL) has received approval from the Food and Drug Administration for its first drug, cabozantinib. Now called Cometriq, it is used to treat advanced medullary thyroid cancer once the disease has spread to other organs.  However, in this video, Motley Fool health care analyst David Williamson gives reasons to temper enthusiasm over this approval while keeping an eye on future clinical trial results for cabozantinib in prostate cancer.