A Senate committee is investigating whether practices at five of the top makers of opioids in the United States fueled an epidemic of painkiller abuse that has led to the fatal overdoses of tens of thousands of Americans.
Sen. Claire McCaskill, D-Mo., on Tuesday sent letters to the companies seeking information about sales and marketing materials, internal studies on addictions, details on their compliance with legal settlements and donations to advocacy groups.
The companies are Purdue Pharma, Johnson & Johnson's Janssen division, Insys, Mylan and Depomed. McCaskill is the ranking Democrat on the Senate's Homeland Security and Governmental Affairs Committee.
By late morning, Depomed's stock was down 3.3 percent; Insy shares were down 1.8 percent and J&J was down about 0.5 percent. Mylan's price rose 0.6 percent.
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The companies targeted are the makers of the top five opioid products by sales.
"The investigation will explore whether pharmaceutical manufacturers — at the head of the opioids pipeline — have contributed to opioid over-utilization and over-prescription as overdose deaths in the last 15 years have approached nearly 200,000," said a press release announcing the probe.
The statement notes that in 2015 alone, more than 30,000 people died from overdoses of either prescription opioids or of heroin.
And since 1999, the release says, sales of prescription opioids have increased four-fold.
"This epidemic is the direct result of a calculated sales and marketing strategy major opioid manufacturers have allegedly pursued over the past 20 years to expand their market share and increase dependency on powerful — and often deadly — painkillers," McCaskill wrote in her letters to the companies.
"To achieve this goal, manufactures have reportedly sought, among other techniques, to downplay the risk of addiction to their products and encourage physicians to prescribe opioids for all cases of pain and in high doses," she wrote.
McCaskill said she often hears stories about "drug overdose deaths, the vast majority of them related to prescription opioids or heroin," which are "single-handedly destroying families and communities across Missouri and the country."
"And I refuse to just stand by and watch. We have an obligation to everyone devastated by this epidemic to find answers," McCaskill said. "All of this didn't happen overnight. It happened one prescription and marketing program at at time."
Her letters note that in 2007, Purdue Pharma paid $635 million in fines to settle criminal and civil charges related to the company's misrepresentation of the addictive qualities of its OxyContin painkiller medication, and that three executives of the firm pleaded guilty to criminal misbranding.
They also note that the city of Chicago sued Purdue Pharma, Janssen, and several other opioid makers in 2015, claiming the companies deceptively marketed their painkillers.
The Chicago lawsuit alleges that Purdue and Janssen have sponsored continuing medical education activities for doctors "that were delivered thousands of times promoting chronic opioid and supporting and disseminating ... deceptive and biased messages."
The sessions "inflate the benefits of opioids, and frequently omit or downplay their risks or adverse effects," the complaint claims.
Purdue Pharma and Janssen have denied the claims.
Purdue Pharma, in response to McCaskill's letter, told CNBC, "The opioid crisis is among our nation's top health challenges, which is why our company has dedicated itself for years to being part of the solution."
"OxyContin accounts for only 2 percent of the opioid analgesic prescription market nationally, but we are an industry leader in the development of abuse-deterrent technology and advocating for the use of prescription drug monitoring programs," the company said. "We are reviewing Senator McCaskill's letter and will respond accordingly."
Janssen spokeswoman Jessica Castles Smith said, "We received the letter from Senator McCaskill and we plan to address her request."
"We believe that we have acted appropriately, responsibly and in the best interests of patients regarding our opioid pain medications, which are FDA-approved and carry FDA-mandated warnings about the known risks of the medications on every product label," said Smith.
In response to the letter Mylan said: "We welcome the senator's interest in this important matter and we share her concerns regarding the misuse of prescription opioids. We very much hope that the senator expands her interest to include the top 10 suppliers of opioid drugs to the U.S. market. In 2016, Mylan ranked as the No. 17 supplier of opioids sold in the U.S., representing approximately 1 percent of the entire U.S. opioid market. Despite being a small player in this area, we are committed to helping find solutions to the issue of opioid abuse and misuse."
Depomed confirmed receipt of the letter from McCaskill, but had no immediate comment. Insys has not yet responded to CNBC for comment.
— Additional reporting by CNBC's Natalia Wojcik.