Modernizing Drug Enforcement Act of 2019 This bill deems a drug or substance that acts as an opioid mu receptor agonist (e.g., morphine) to be in schedule I of the Controlled Substances Act, subject to certain exceptions. A schedule I controlled substance is a drug, substance, or chemical that has a high potential for abuse; has no currently accepted medical value; and is subject to regulatory controls and administrative, civil, and criminal penalties under the Controlled Substances Act.