Perfect competition and monopoly

Under perfect competition, there are price takers that can only accept marekt price. They will produce and sell the quantity of goods where marginal revenue (price) equals marginal cost. This is the most efficient way to allocative resources (pareto optimal). However, monopoly even will produce at the volume where marginal revenue equals marginal cost, but the selling price would be higher than the marginal cost and deadweight loss resulted. This is not efficient in long run.