Large companies in the health care space, such as drugmaker Merck (MRK 0.15%), are typically insulated from general secular trends and tend to make good defensive plays for long-term investors. However, this rule of thumb should be taken with a grain of salt; not every pharmaceutical company is the right fit for your portfolio, and investors always need to consider the bull and bear theses of their investments. In this video, our health care analyst dives into the bear thesis for Merck.