An insulin pump. Alden Chadwick/Flickr A lawsuit filed Monday alleges that the three companies that make insulin have been part of an "organized scheme to drive up prices at the expense of patients who need insulin drugs to live."

Insulin is a hormone that helps people absorb and process the sugar in food. Roughly 1.25 million people in the US who have Type 1 diabetes need to inject insulin to live, as do many people with Type 2 diabetes, the more common form.

The complaint filed in the US District Court of Massachusetts details examples of patients who pay $900 a month for the drug, as well as people who induced diabetic ketoacidosis — a potentially fatal condition in which the body builds up too much of a certain blood acid because there isn't enough insulin in the body — to have access to insulin samples in the emergency room.

The suit alleges that the drug companies — Novo Nordisk, Sanofi, and Lilly — violate the Racketeer Influenced and Corrupt Organizations Act.

"People living with diabetes are practically imprisoned under the price hikes and sadly are resorting to extreme measures to afford the medication they need to live," Steve Berman, a managing partner at the legal firm representing the patients in the suit.

The suit is seeking class-action status.

Here's how the three companies responded: