 On December 16, 2018, an investigative article in The Toronto Star reported apparent problems in more than 400 papers coauthored by Koren, including that the papers "[were] inadequately peer-reviewed, fail to declare, perhaps even obscure, conflicts of interest and, in a handful of cases, contain lies about the methodology.” In 2019 Koren threatened a defamation lawsuit against the editor of "Therapeutic Drug Monitoring" for retracting one of Koren's papers.As of 2020, Koren has had two of his research publications retracted, two others have received an expression of concern, and four others have been corrected.In February 2019, Koren agreed to relinquish his licence to practice medicine in Ontario in the face of an investigation by the College of Physicians and Surgeons into whether he committed “professional misconduct or was incompetent” while he was in charge of the Hospital for Sick Children’s Motherisk laboratory.