Walgreens last week said it was buying Rite Aid. Analysts say the deal could allow the retail chain to demand better terms from big drug makers and pharmacy benefit managers. Whether or not this merger improves Walgreens’s bargaining position, it will reduce consumer choice and may lead to higher prices.

The presence of a few dominant companies in an industry also makes it harder for entrepreneurs to start new businesses in that sector. The rate at which new businesses are created in the economy as a whole has been steadily falling since the 1970s, according to the Census Bureau. In 2013, the growth rate was 10.2 percent, down from 17.1 percent in 1977.

More legal challenges could help limit further consolidation and restore more dynamism to the economy. The Justice Department’s antitrust division has in recent years won nearly every merger challenge it has brought. The division and the Federal Communications Commission prevented Comcast from buying Time Warner Cable and AT&T from acquiring T-Mobile. But the division’s record suggests that it could be taking a tougher line. It allowed American Airlines and US Airways to merge after they agreed to sell just a few gates and take off and landing slots to other airlines. It should not have settled so quickly for so little.

Congress should also study whether there are ways to strengthen the antitrust laws. It could, for instance, require regulators to consider whether mergers in concentrated industries are in the broader public interest, not just whether they would harm consumers through higher prices. The F.C.C. can already use such a test to evaluate media and telecom deals to determine, for example, whether a merger will limit the diversity of opinions in media. Unfortunately, Congress is unlikely to adopt new antitrust legislation while Republicans are in control of both houses.

Given the already high consolidation in many industries, government officials have to be vigilant about investigating businesses that abuse their dominant position. In extreme cases, antitrust laws allow officials to seek the breakup of businesses, as the Justice Department did with the old AT&T monopoly in 1974. (That case was settled in 1982, and the company was broken up in 1984.) But it will be harder to use this tool now because most dominant companies are careful not to assemble that level of market power. That makes it all the more important for officials to do a better job of reviewing mergers in the first place.

Markets work best when there is healthy competition among businesses. In too many industries, that competition just doesn’t exist anymore.

Correction: November 1, 2015

An earlier version of this editorial misattributed a statement about the proposed deal between Walgreens and Rite Aid. It was industry analysts, not Walgreens, who said the merger could allow Walgreens to demand better terms from drug makers and pharmacy benefit managers.