A rush of acquisitions this year is reshaping the drug industry. But investors haven’t universally welcomed the deal-making.

AbbVie’s agreement on Tuesday to buy Allergan, in a deal valued at $63 billion, provided the latest evidence: AbbVie’s shares fell 16 percent, their worst day in six years. Months earlier, investors pushed the stock of Bristol-Myers Squibb down 14 percent after the announcement that it would buy Celgene for $74 billion, the year’s biggest pharmaceutical deal.

In both deals, the rationale for the companies — as well as the immediate negative reaction in the markets — reflect the industry’s dependence on blockbuster drugs to drive revenue growth, and the vulnerability of the companies when those drugs lose patent protections.

It’s a dynamic that has pushed drugmakers to make deals. Both Celgene and AbbVie have best-selling drugs that will lose patent protection in the next five years.