Perfect competition is allocatively efficient. Monopoly is productively efficient. Monopoly is not allocatively efficient. The difference between the price that the firm gets for its good and the lowest price it would be willing to accept. There are several causes of market failure in terms of markets failing to achieve the efficient outcome.
Monopoly is one cause of market failure.
One of the issues in public policy is whether and under what circumstances government action can increase the allocative efficiency of market outcomes. The government has two types of policies for dealing with monopoly practices the set of noncompetitive practices. Antitrust policy refers to the set of laws and other instruments used to encourage competition and discourage monopoly practices. The purpose of this policy has been to create more competitive market structures, and when that is not possible to discourage monopolistic practices and encourage competitive practices. Regulation refers to the rules under which firms can do business and in some cases determines the prices they can charge.