A lawsuit filed Monday accused three makers of insulin of conspiring to drive up the prices of their lifesaving drugs, harming patients who were being asked to pay for a growing share of their drug bills.

The price of insulin has skyrocketed in recent years, with the three manufacturers — Sanofi, Novo Nordisk and Eli Lilly — raising the list prices of their products in near lock step, prompting outcry from patient groups and doctors who have pointed out that the rising prices appear to have little to do with increased production costs.

The lawsuit, filed in federal court in Massachusetts, accuses the companies of exploiting the country’s opaque drug-pricing system in a way that benefits themselves and the intermediaries known as pharmacy benefit managers. It cites several examples of patients with diabetes who, unable to afford their insulin treatments, which can cost up to $900 a month, have resorted to injecting themselves with expired insulin or starving themselves to control their blood sugar. Some patients, the lawsuit said, intentionally allowed themselves to slip into diabetic ketoacidosis — a blood syndrome that can be fatal — to get insulin from hospital emergency rooms.

A recent study in The Journal of the American Medical Association found that the price of insulin nearly tripled from 2002 to 2013.

“People who have to pay out of pocket for insulin are paying enormous prices when they shouldn’t be,” said Steve Berman, a lawyer whose firm filed the suit on behalf of patients and is seeking to have it certified as a class action.

In a statement, Sanofi said, “We strongly believe these allegations have no merit, and will defend against these claims.” Lilly said it had followed all laws, adding, “We adhere to the highest ethical standards.”

A spokesman for Novo Nordisk said the company disagreed with the allegations in the suit and would defend itself. “At Novo Nordisk,” the company’s statement said, “we have a longstanding commitment to supporting patients’ access to our medicines.”

The rising costs of drugs has led to several hearings in Congress and has drawn the attention of President Trump, who this month pledged to address the issue and said the industry was “getting away with murder.”