Pills with this imprint are semi-synthetic opioid painkillers mixed with acetaminophen, the active ingredient in Tylenol. Specifically, an oblong white pill with M366 stamped on it contains 7.5mg of the opioid Hydrocodone and 325mg of acetaminophen while one with the imprint M367 has 10mg of Hydrocodone and 325mg of acetaminophen and contains an even higher opioid dose.