Is Merck Stock a Buy?

There is mounting pressure on the company's most prized franchise.Anyone crafting a bull case for the pharmaceutical giant Merck (MRK 0.53%) should include Keytruda as the company's best asset. The cancer drug has been Merck's top-selling medicine and its biggest growth driver -- by a mile. And last year, it became the best-selling drug worldwide, taking over from the immunology superstar Humira. However, Keytruda won't hold that title forever. What will happen if Merck's most important cash cow runs into s ...