A high speed production line of insulin is pictured at the factory of Novo Nordisk, a global healthcare company, in Chartres.

Democratic leaders sent letters Wednesday to three major pharmaceutical companies asking for the "root causes" of rising insulin costs, the latest attempt by Democrats to tackle high health-care costs in the U.S.

Energy and Commerce Committee Chairman Frank Pallone, Jr, D-Nj, and Oversight and Investigations Subcommittee Chair Diana DeGette, D-Co, sent letters to Eli Lilly, Novo Nordisk and Sanofi on Wednesday requesting information on insulin prices and the obstacles to providing more affordable medication. The Democrats are requesting answers from each of the companies by Feb 13.

"When patients go without insulin — or ration their doses — there can be tragic consequences," Pallone and DeGette wrote in their letters. "As one of the few manufacturers of insulin in the United States, your company is well-suited to shed light on these issues and offer potential solutions."

CNBC reached out to Eli Lilly, Novo Nordisk and Sanofi for comment.

A spokesperson for Sanofi confirmed the company received a letter and said it will work with the committee on their request for information.

Earlier this month, House Oversight Committee Chairman Elijah Cummings, D-Md., said he sent letters to 12 drugmakers seeking detailed information and documents about the companies' pricing practices. Eli Lilly, Novo Nordisk and Sanofi were among the 12 which received letters.

Democrats, who regained control of the House this month, listed lowering prescription drug costs as one of their top priorities. Spending on prescription drugs in the U.S. increased to $333.4 billion in 2017, according to the latest data from the Centers for Medicare and Medicaid Services. Additionally, the annual cost of insulin for people with type 1 diabetes in the U.S. nearly doubled from 2012 to 2016 to $5,700 from $2,900.

Democrats have placed their focus on the pharmaceutical industry, which sells drugs at higher prices in the U.S. than abroad.

The Trump administration has also vowed to lower drug costs, and President Donald Trump has indicated he's willing to work with Democrats to do so.

In a brief exchange between Rep. Mark Meadows and Cummings at a hearing Tuesday, the North Carolina Republican said Trump wanted Cummings to know Trump's "serious about working in a bipartisan way to lower prescription drug prices."

Pharmaceutical companies say price increases have been modest and argue lowering drug profits would hinder the industry in making investments towards new and innovative cures.

