WASHINGTON — Just about anyone who has gone to a pharmacy and paid for a prescription knows that a generic copy costs much less than the brand-name drug. The makers of those two versions of a drug, therefore, usually compete fiercely for market share and profits.

But at the Supreme Court on Monday, the generic and the brand-name drug companies will be on the same side, arguing against the federal government in the legal equivalent of a heavyweight title bout.

The case, Federal Trade Commission v. Actavis, No. 12-416, centers on whether the maker of a brand-name drug can pay a generic-drug company to keep the generic version off the market. Based on antitrust law, the obvious answer would seem to be no, the view voiced by the government and most recently upheld by a federal appeals court.

At least three other federal appeals courts have previously said those payments are legal, however, when made under the settlement of a patent infringement lawsuit. Those courts sided with drug company arguments that the payments are what Congress intended in setting up guidelines to encourage the production of generic drugs.