n June 2016, Pfizer Inc. raised the list price of its drug Viagra by 13%. Less than a week later, Eli Lilly & Co. increased the price of its similar pill, Cialis, by 13%. That was not the first time...

n June 2016, Pfizer Inc. raised the list price of its drug Viagra by 13%. Less than a week later, Eli Lilly & Co. increased the price of its similar pill, Cialis, by 13%. That was not the first time Lilly followed a Pfizer price increase. As the figure shows, these two drugs’ prices have been moving in lockstep roughly twice annually for several years. As a result, the list price has more than doubled over the past five years. The prices of competing insulin, bloodthinner, and other drugs have also moved together. This practice has contributed to the surge in prescription drug spending over recent years. As a result, legislators have been calling upon the Justice Department and the Federal Trade Commission to investigate possible collusion. However, as long as the firms do not explicitly communicate, such signaling does not violate antitrust laws.