In the following video, Fool contributor Maxxwell Chatsko provides an update on the outlook of Merck's best-selling type 2 diabetes drug franchise of Januvia/Janumet. While the franchise reeled in about $5.75 billion in 2012, it stumbled out of the gates in 2013 and surprised investors with declining overall sales. Merck has been hit especially hard by the patent cliff, so this could become a worst-case scenario for investors. Maxxwell cautions that while the slide may be temporary, investors absolutely need to know and watch two key areas in the rest of 2013.