WASHINGTON – For more than a decade, drugmakers have battled Federal Trade Commission accusations that they have colluded to forestall the introduction of dozens of generic medicines, costing buyers as much as $3.5 billion a year.

The fight hits a climax March 25 when the Supreme Court hears arguments on what the FTC calls “pay for delay” patent settlements. Under those accords, the FTC says, a brand-name drugmaker pays a would-be rival to push back the release of a low-cost generic medicine. Drug companies say the deals have the opposite effect, clearing the way for generic competition by resolving patent issues.

The FTC is seeking a ruling that those deals are generally anticompetitive, a decision that could alter the landscape for U.S. drug sales and lead to a wave of suits by wholesalers, retailers and insurers.

Bayer, Merck & Co. and Bristol-Myers Squibb already have faced suits, and the agency says 40 more pay-for-delay accords were struck in fiscal 2012 alone.

“I cannot emphasize enough the enormous consequences with respect to this Supreme Court decision,” said Ralph Neas, president of the Generic Pharmaceutical Association, a Washington industry trade group that opposes the FTC campaign.

The Supreme Court argument is one of two this month that will affect the generic-drug industry. On Tuesday the court will weigh limits on patient lawsuits, using the case of a woman awarded $21 million for injuries she incurred from a painkiller made by Takeda Pharmaceutical Co.

Patients are seeking a way around a 2011 Supreme Court ruling that shielded generic-drug companies from claims that they failed to warn of possibly dangerous side effects. A federal appeals court said the patient could sue because she was pressing a different legal theory, claiming the drug was so dangerous it shouldn’t have been on the market.

The high court will rule in both cases by June.

The disputed patent settlements are a product of the economics of the pharmaceutical industry, where companies can reap billions of dollars from blockbuster drugs – and then see those sales plummet the moment a generic alternative appears. The FTC says generic drugs sell for an average of 15 percent of the original price, with the brand-name company losing 90 percent of its market share by unit sales. Generics have saved purchasers $1.1 trillion in the last decade, Neas says.

Pharmaceutical patent settlements typically arise when a generic-drug maker has either secured, or is poised to secure, Food and Drug Administration approval. At that stage, only the brand-name company’s patents stand in the way of generic competition.

The FTC and its allies say they have no quarrel with settlements that merely set the date for generic entry. They say that type of agreement simply reflects the companies’ assessments of the chances that a court would invalidate the brand-name company’s patent.

A payment is different, they say. If a brand-name drugmaker with $100 million in annual sales can pay a generic rival $20 million to wait an extra year, both companies come out ahead – at the expense of purchasers, the FTC argues.

“It is a win-win for the pharmaceuticals, and it is a lose-lose for the consumers,” said Jon Leibowitz, who stepped down this month as the FTC’s chairman after making drug-industry settlements his signature issue. “The brands and generics are conspiring to prevent competition, and the consumers are essentially the ones who pay the premium for that.”

A 2010 FTC study found that the accords cost purchasers $3.5 billion a year, a figure the drug industry contests.