BOSTON—A unit of French advertising company Publicis Groupe SA on Friday lost a bid to dismiss a lawsuit by Massachusetts’ attorney general claiming it helped OxyContin maker Purdue Pharma LP fuel the U.S. opioid epidemic. Publicis Health had called Attorney General Maura Healey’s lawsuit an unprecedented attempt to sue an advertising agency over a manufacturer’s marketing of its products. It called her allegations conclusory and said she mischaracterized documents. But Suffolk County Superior Court Judge Brian Davis said Healey brought “non-speculative” claims under the state’s public nuisance law and consumer protection statute that could move forward. He pointed to allegations in Healey’s complaint, filed in May, that he said showed Publicis “played an integral part in developing marketing strategies” to boost opioid sales from 2010 to 2019. Davis said those marketing campaigns were designed to get doctors “to use more OxyContin, prescribe higher doses and prescribe it for longer periods …