At least eight pharmaceutical companies sell a decades-old drug that treats gallstones, but the competition has done little to keep its price down.

Instead the price has skyrocketed.

Two years ago, ursodiol's wholesale price was as low as 45 cents a capsule. Then in May 2014, generic drug manufacturer Lannett Co. hiked its price to $5.10 per capsule, and one by one its competitors followed suit — with most charging nearly the same price.

Experts say this is not how a competitive marketplace is supposed to work.

“When you have a generic drug with eight suppliers, you would expect the prices to go down,” said Dana Goldman, director of USC's Leonard D. Schaeffer Center for Health Policy & Economics.

Unlike nearly every other developed nation, the U.S. allows drug manufacturers to set their own prices, a policy that has resulted in overall medicine costs being far higher than elsewhere. Increasingly, insurers are passing the cost along to patients through higher deductibles.

Robert Frankil, the owner of Sellersville Pharmacy in Pennsylvania, said ursodiol is just one of dozens of generic drugs that he has found to spike in price in the last couple of years.

“Why are these companies raising their prices?” asked Frankil. “Because they can.”

He said many of his patients in high-deductible plans end up paying full price.

“Patients paid $40 for their prescription one month and $400 the next,” Frankil said. “Nobody can believe this is happening.”

One of the ursodiol suppliers is Mylan, which recently stirred outrage with its steep price hikes of another medicine, the EpiPen device, just as children went back to school. The device automatically injects a drug called epinephrine to counteract life-threatening allergic reactions.

Since buying EpiPen from another company in 2007, Mylan has continually raised its wholesale price, often called the list price. The price rose from $94 in 2007 to $608, a rise of 547 percent, according to data from Truven Health Analytics.

Some experts have blamed the EpiPen price hikes on a lack of competition. But even when Sanofi, a competitor, introduced another automatic epinephrine injector in 2013 to challenge Mylan, it charged exactly the same price — $241 for a package of two.

The two companies then continued to repeatedly raise their prices until 2015, when Sanofi took its device called Auvi-Q off the market because it may have been inaccurately delivering the drug. At that time, both companies were charging about $500.

The skyrocketing prices of EpiPen and ursodiol show why prescription medicines are making up an ever greater share of health spending.

According to the federal Health and Human Services Department, prescription drugs now account for almost 17 percent of personal healthcare expenditures — up from about 7 percent in the 1990s.

Lannett, which is headquartered in Philadelphia, has detailed in its financial statements how its price increases on ursodiol and other medicines have boosted sales and profits.

The company said that the price of its gallstone medicines, including ursodiol, rose by 907 percent in the year ended June 30, 2015, adding $58.7 million to sales.

Overall, the company said, price hikes on myriad medicines accounted for 39 percent, or $157.3 million, of its net sales of $406.8 million.

Lannett executives said this week that they could not comment because of an investigation by the U.S. Justice Department into their pricing practices. Prosecutors are looking at possible violations of the Sherman Antitrust Act, which outlawed monopolistic business practices.

In December 2014, the company told shareholders that it had received a grand jury subpoena requesting information that included communications with competitors about the pricing and sale of certain products.

Other companies raising their wholesale price of ursodiol to more than $5 for a 300-milligram capsule include Epic Pharma, Teva Pharmaceuticals, Avkare, Marlex Pharmaceuticals and Major Pharmaceuticals. The fact that the price hike was led by generic-drug companies seems to turn the industry's traditional price model on its head.

Mylan stopped selling ursodiol in 2012 and then brought it back in January, charging a wholesale price of $4.95 per capsule.

Nina Devlin, a Mylan spokesman, said ursodiol's price increase happened while the company was out of the market.

She said Mylan reintroduced the drug in January at what had become the “current market price.”

Michele Pelkowski, a spokeswoman for Israel-based Teva, said the price hike had been made by Actavis Generics, which Teva recently acquired. She said Teva was aware of a shortage of active ingredients needed to manufacture ursodiol that may have affected the market.

Cliff Stanfill, an executive at Avkare in Pulaski, Tenn., which sells medicines to the federal government, said, “It's our policy not to speak about pricing to anyone except our buyers.”