States can challenge proposed hospital mergers by using antitrust laws to prevent anticompetitive harms. This observational study examined additional state laws—principally charitable trust, nonprofit corporation, health and safety, and certificate-of-need laws—that can serve as complements and substitutes for antitrust laws by empowering states to be notified of, review, and challenge proposed hospital mergers through administrative processes. During the period 2010–19, 862 hospital mergers were proposed, but only forty-two (4.9 percent) were challenged by states, including thirty-five by states without federal involvement, of which twenty-five (71.4 percent) originated in the eight states with the most robust merger review authority. The twenty-five challenges resulted in two mergers being blocked; three being abandoned; and twenty being approved with conditions, including seven with competitive-impact conditions. Hospital market concentration and prices increased at similar rates in these eight states versus other states, potentially because most challenges allowed mergers to proceed with conditions that did not adequately address competitive concerns. Although these findings do not reveal an optimal state framework, elements of advanced state merger review authority may have the potential to improve poorly functioning hospital markets.