Imagine that your company has agreed to a multibillion-dollar acquisition of a competitor. After spending millions in fees and successfully navigating the antitrust process, you are on the verge of closing. Then, at the eleventh hour, a group of customers sues to block the deal and asks the court to stop the closing.

This scenario is not fiction. It has occurred at least 12 times since 2007, including in Pfizer Inc.’s $68 billion acquisition of Wyeth, InBev’s $52 billion acquisition of Anheuser-Busch, Sirius’s $13 billion merger with XM Radio, United Air Lines Inc.’s $3.2 billion acquisition of Continental Airlines Inc., and Southwest Airlines Co.’s $1.4 billion acquisition of AirTran Airways Inc. So far, no court has ordered an injunction, but one case, Blessing v. Sirius XM Radio (2011), resulted in a settlement with consumers in which the combined Sirius XM Radio agreed not to raise prices.

When planning any large transaction, counsel needs to evaluate the likelihood of a private challenge—and consider the steps to take to reduce the chances of a suit and how to be best positioned if it occurs.

Who is likely to sue?   

Private antitrust suits brought under Section 7 of the Clayton Act can come from competitors, distributors, suppliers, or even targets, but recent suits have generally come from parties’ customers. Consumers pose the greatest threat to a deal. Because they are the group that the antitrust laws are designed to protect, they face the fewest impediments. In particular, consumers can easily establish antitrust standing—injury caused by a loss of competition—because they suffer harm when a transaction has anticompetitive effects, such as higher prices.

Which deals are most susceptible to private challenges?