Patients are facing a sudden increase in costs for generic drugs, which is putting their health at risk, according to a report by the New York Times.

Prices for digoxin, a drug used primarily by older patients to treat rapid heart rhythm problems and heart failure, have spiked sharply in recent months.

Surveyed pharmacies had often tripled the drug’s costs from last October to June, said Doug Hirsch, chief executive of GoodRx.com, to the Times. He added that some patients reported seeing costs of more than $1,000, which can lead to hundreds of dollars in co-pay.

Alternatives to digoxin, and so market competition, are scarce. In 2013, only two businesses — Lannett Company and Global Pharmaceuticals — dominated the market after several companies stopped manufacturing the drug. Prices that year soared after one company increased its fees and the others followed suit, said the report.

Following the Times’ request for comment, Lannett wrote in an email statement, “On occasion and for a variety of reasons generic drug makers can and do raise prices.”

Whatever these reasons, they result in sudden profits. Investors told the Times that sales of Lannett’s cardiovascular drugs rose from $4.5 million to $16.9 million within just a few months.

Digoxin isn’t the only drug to be hit by price hikes. EvaluatePharma found that the cost of fiorinal with codeine, synthroid as well as the generic steroid prednisolone have doubled since 2013. In January, the National Community Pharmacists Association called for a congressional hearing to address the spiraling cost of essential generic drugs, which have risen “600%, 1,000% or more” in the past few years.

“The skyrocketing cost of scores of generic drugs is harming patients and community pharmacies and raises significant questions that deserve to be examined in a congressional committee hearing,” the association said in a statement.