(CNN) A new report alleges that five of the United States' largest opioid manufacturers paid patient advocacy groups nearly $9 million between 2012 and 2017 to help push an agenda that promoted opioid use.

McCaskill's office investigated the marketing and sales practices of the pharmaceutical companies Purdue Pharma LP, Janssen, Mylan, Depomed and Insys based on their roles in manufacturing some of the opioid painkillers with the highest sales in 2015.

According to the new report, 14 patient advocacy groups, including the Academy of Integrative Pain Management and the US Pain Foundation, received $8,856,339 in payments from the pharmaceutical companies over five years. The US Pain Foundation received the largest share of payments, totaling nearly $3 million.

The report found that the Academy of Integrative Pain Management and the American Academy of Pain Medicine each collected about $1.2 million in that same period.

The most generous payer was Purdue Pharma LP, which provided nearly half the manufacturers' almost $9 million in payments. In the five years tracked, the report found, its payments to outside advocacy groups totaled just over $4.1 million.

On Sunday, Purdue, which has been criticized for its aggressive marketing of the painkiller OxyContin, said it will no longer send sales representatives to doctors' offices to promote the drug.

In response to Monday's report, Purdue noted in a statement its "significant sales force reduction of more than 50%" and said those remaining on the job will be promoting "non-opioid products."

Purdue acknowledged the donations, but Executive Director for Communications Robert Josephson said in a statement, "We have supported third-party organizations, including with annual dues and unrestricted grants, that are interested in helping patients receive appropriate care.

Christine Dusek, head of global communications at Mylan, said in a statement, "Over the past few years, Mylan made very limited payments to the American Pain Society solely as part of its participation in the organization's annual conferences. We did not sponsor or fund any speakers or presentations at these conferences."

Depomed said in an email that it made contributions to nine of 14 foundations identified in the Senate committee report, totaling $1.07 million over six years.

"These contributions covered corporate advertising, conference booth fees, sponsoring training certifications and membership fees," said Christopher Keenan, vice president for investor relations and corporate communications. He added that Depomed believes it acted responsibly.

Insys also confirmed that it made such payments but characterized them in a statement as "patient focused."

"With regard to the US Pain Foundation specifically, our donation was directed to a disease-state fund for cancer patients with breakthrough pain, which many medical experts and health care providers believe is significantly undertreated in the US." The statement specified that such "charitable contributions" are allowed because they benefit patients.

The company also noted a 77% decrease in contributions to these groups during the second half of 2017 as part of a shift to focus more on research and development.

The manufacturers who offered responses to the report said they had not seen it in its entirety.

Payments to physicians

On top of payments directly to the advocacy groups, the investigation found that $1.6 million was paid to physicians affiliated with these groups since 2013.

In a statement, McCaskill said, "The pharmaceutical industry spent a generation downplaying the risks of opioid addiction and trying to expand their customer base for these incredibly dangerous medications and this report makes clear they made investments in third-party organizations that could further those goals."

The investigation noted that the patient organizations were heavily critical of the US Centers for Disease Control and Prevention's opioid-prescribing guidelines to look at alternatives, published in 2016.

For example, Monday's report noted that the past president of the American Academy of Pain Medicine, Dr. Daniel Carr, said, "The CDC guidelines makes disproportionately strong recommendations based upon a narrowly selected portion of the available clinical science." Other groups were cited for trying to undermine the CDC's process by calling the guidelines "biased" and a result of "conflicts of interest."

Josephson said Purdue agrees with the CDC guidelines for prescribing opioids and has been recommending that they be followed since they were issued.

Lobbying efforts

The report also found that advocacy groups have been involved in lobbying efforts to defeat legislative measures to limit prescribing. The investigation offered a 2014 example in which the Academy of Integrative Pain Management teamed up with the American Cancer Society Cancer Action Network to protect a 2001 law in Tennessee that made it difficult to discipline doctors who may have been overprescribing opioids.

Academy of Integrative Pain Management Executive Director Bob Twillman said "there is no proof that the [Tennessee] law in question ever prevented anyone from being either prosecuted or having disciplinary action taken by a licensing board." He added that the law was "poorly written" and said "it was much more of an imagined impediment than a real impediment. The problem was that those wanting to repeal the law misrepresented what it actually said when debating the bills in the legislature."

Twillman criticized Monday's report -- which he said he had seen only parts of -- as being limited. "It appears that they've simply looked at how much money we got from a set of pharma companies, constructed a narrative about what that means and published it." Twillman said that's only half the story, adding that his group and others like it are part of the solution to the opioid crisis.

"The fact that these groups registered their opposition while receiving funding from the opioids industry raises the appearance -- at the very least -- of a direct link between corporate donations and the advancement of opioids-friendly messaging," the new report states.

Dusek said Mylan's priority is patient safety and noted that opioid-containing medications are a small fraction of the medications it manufactures, amounting to approximately 1% of opioids sold in the United States in 2016. She noted that the company has not seen the report in full, but criticized what she had seen so far as political.

"Lumping Mylan in with this group of companies is not only highly irresponsible, but also highlights more of a political agenda rather than finding real solution to the opioids crisis," said Dusek. "Mylan made very limited payments to only one of the 14 organizations outlined in the report for a total of $20,250 over three years, translating to 0.2% of the $10 million in the executive summary. [McCaskill] chose to ignore these facts that differentiate Mylan from the other companies," said Dusek.

Follow CNN Health on Facebook and Twitter See the latest news and share your comments with CNN Health on Facebook and Twitter.

Janssen, a Johnson & Johnson company, said, "While we cooperated with the senator and provided her staff with the requested information, we have not yet received a copy of the final report and can't comment on it's content."

According to the investigation committee, the donors to these advocacy groups did not have to be disclosed by law because of the groups' nonprofit status.

"The financial relationships between these groups and opioid manufacturers should be clear to the general public," McCaskill said. "We passed a law ensuring the public had information on payments to doctors by pharmaceutical companies, and I can't imagine why the same shouldn't be done in this space."