Jeff Smith, a partner with the influential consulting firm McKinsey & Company, accepted a highly sensitive assignment in December 2017. The opioid manufacturer Purdue Pharma ... sought out Dr. Smith. His team reviewed business plans and evaluated new drugs that Purdue hoped would help move the company beyond the turmoil associated with OxyContin, its addictive painkiller that medical experts say helped to spark the opioid epidemic. But the corporate reorganization was not Dr. Smith's only assignment. He was also helping the Food and Drug Administration overhaul its office that approves new drugs the same office that would determine the regulatory fate of Purdue's new line of proposed products. A review ... of internal McKinsey documents found that the firm repeatedly allowed employees who served pharmaceutical companies, including opioid makers, to also consult for the F.D.A., the drug industry's primary government regulator. And, the documents show, McKinsey touted that inside access in pitches to private clients. In an email in 2014 to Purdue's chief executive, a McKinsey consultant highlighted the firm's work for the F.D.A. and stressed "who we know and what we know." McKinsey also allowed employees advising Purdue to help shape materials that were intended for government officials and agencies, including a memo in 2018 prepared for Alex M. Azar II. References to the severity of the opioid crisis in a draft version of the memo ... were cut before it was sent to Mr. Azar.