Sen. Bernie Sanders (I-Vt.) and a group of Senate Democrats on Monday announced an investigation into the high costs of asthma inhalers.

Sanders, chairman of the Senate Health, Education, Labor and Pensions (HELP) Committee, sent letters to the CEOs of the four biggest manufacturers of inhalers sold in the United States — AstraZeneca, Boehringer Ingelheim (BI), GlaxoSmithKline (GSK) and Teva — demanding information and documents on internal strategic communications, patient assistance programs and the costs involved in the manufacturing of inhalers.

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Like many other prescription drugs, the companies’ inhalers sell for hundreds of dollars in the U.S. but for only fractions of that same cost overseas. The companies charge between $200 and $600 each for inhaler products that are typically purchased monthly.

For example, one of AstraZeneca’s inhalers costs $645 in the U.S. but just $49 in the U.K. One of Boehringer Ingelheim’s inhalers costs $489 in the U.S. but just $7 in France. GSK’s popular Advair HFA costs $319 in the U.S. but just $26 in the U.K.