By Patrick Wingrove

Aug 31 (Reuters) - Two U.S. senators are demanding that the nation's three largest insulin makers, Novo Nordisk , Eli Lilly, and Sanofi, provide details by Sept. 15 of their programs to help Americans get their insulin for $35 a month or less.

Democratic Senators Maggie Hassan and Tina Smith sent a letter asking the drugmakers for information on eligibility criteria for the programs, including whether a patient’s insurance status or income barred them from joining, and the steps insulin users had to take to sign up.

Novo, Lilly and Sanofi, which account for 90% of the U.S. insulin market, pledged in March to lower the list prices of many of their insulin products by 70%-78% later this year or in 2024. The Biden administration and some lawmakers have continued to press them to lower the costs of the medicines.

The White House on Tuesday announced that a Novo insulin was among the 10 high-cost prescription drugs selected for the first-ever price negotiations by the U.S. Medicare health program that covers 66 million people.

In July, Senator Elizabeth Warren, a Democrat, released a report showing that pharmacies were charging as much as $330 for Lilly’s generic insulin, despite the drugmaker having lowered the cost to $25 a vial. Lilly said it did not control how much pharmacies demand for its drugs.

Hassan and Smith argued that to enroll in the drugmakers’ insulin programs, patients had to go through a lengthy and complicated process that required them fill out five to 10 pages of documentation and wait an unknown amount of time for approval.

The senators asked for copies of the applications for each of the companies' programs, for details of any additional steps patients had to take to join, and average wait times before approval, as well as what personal and medical information patients had to hand over.

Around 8.4 million of the 37 million people in the United States with diabetes use insulin, according to the American Diabetes Association. (Reporting by Patrick Wingrove Editing by Bill Berkrot)