Four U.S. senators asked the Federal Trade Commission to investigate whether suppliers of saline solution have illegally inflated prices to exploit a chronic shortage of the hospital staple.

The senators sent a letter Monday to FTC Chairwoman Edith Ramirez stating that prices for saline have risen 200% to 300% since a shortage began in late 2013. The letter was signed by two Democrats— Richard Blumenthal of Connecticut and Amy Klobuchar of Minnesota-—and two Republicans from Utah— Mike Lee and Orrin Hatch.

Saline, also known as injectable sodium chloride, is commonly given intravenously to hydrate patients or to dilute medicines. The U.S. Food and Drug Administration last year said the shortage was a serious threat to patients, and attributed it to increased demand by hospitals. The FDA allowed some manufacturers including B. Braun Medical Inc. and Baxter International Inc. to temporarily import saline from foreign factories to alleviate the shortage.

According to the senators’ letter, the three major suppliers of saline for the U.S. are Baxter, B. Braun and Pfizer Inc.’s Hospira unit.

Baxter disputed that it has raised saline prices that much. A Baxter spokeswoman said the average U.S. selling price for Baxter’s one-liter sterile saline solution has increased annually in the single- to low-double-digits on a percentage basis since late 2013, and Baxter has taken steps to boost its supply.

A Pfizer spokesman said Hospira expanded its production of saline products when the shortage emerged in 2013, and at current prices the solutions are “one of health care’s greatest values.”

A B. Braun spokesman couldn’t be reached for comment.

“While all three companies have publicly spoken to their commitment to ending the saline shortage, it has persisted for two years even as prices have risen,” the senators wrote. The letter said that while prices tend to increase during a shortage, “these price increases appear to be outside the bound of natural market forces.”

The letter also says the saline suppliers are imposing even greater price increases on customers that don’t also purchase other products, like the tubes and catheters through which saline is administered to patients.

“We urge the FTC to formally investigate whether the saline suppliers’ apparent anticompetitive conduct is harming consumers and running afoul of the antitrust laws,” the senators wrote.

Erin Fox, director of the University of Utah Health Care’s Drug Information Service, said the saline supply has improved this year but the shortage hasn’t been resolved.

Saline and other IV solutions are among a rash of drugs in short supply in the U.S. The shortages often force doctors to delay treatments or switch patients to suboptimal alternatives for diseases ranging from bladder cancer to pneumonia. Many of the shortages are caused by manufacturing problems.