Democrats from the House Committee on Oversight and Government Reform have sent letters to seven drugmakers as part of an investigation into why prices for multiple sclerosis drugs have nearly quadrupled since 2004.

Reps. Elijah Cummings (D-Md.) and Peter Welch (D.-Mass) penned inquiries to Bayer (ETR:BAYN), Biogen (NSDQ:BIIB), EMD Serono, Novartis (NYSE:NVS), Roche (OTC:RHHBY), Sanofi Aventis (NYSE:SNY) and Teva Pharmaceuticals (NYSE:TEVA).

In these letters, the U.S. representatives cited a study which claims that some drugmakers appear to be hiking the prices of their products in lockstep with competitors – a practice known as “shadow pricing.”

In other words, when one company launches a new, more expensive product, other companies will shadow these higher prices by hiking their pricetag to match.

The lawmakers said that between 2004 and 2015, the average cost of MS therapy jumped from $16,050 to $60,000.

“We are launching an in-depth investigation to determine why drug companies are dramatically increasing their prices for drugs used to treat Multiple Sclerosis (MS), which is a disease of the central nervous system that often has devastating and disabling effects on patients,” Cummings and Welch wrote.  “We believe no American should be forced to struggle to afford lifesaving medical treatments, especially when drug companies increase prices without warning, cause, or justification.”

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