The SIMSA Act of 2024 creates a new "Schedule A" category in federal drug law to specifically target synthetic drug analogues that are imported or offered for import into the United States. These are substances with chemical structures similar to existing controlled drugs that produce comparable effects on the central nervous system, but may not fit existing drug schedules. The Attorney General can temporarily schedule substances in Schedule A for up to five years, with the option to make permanent scheduling after three years, though the Health and Human Services Secretary can recommend against permanent scheduling based on scientific evidence. The bill establishes significant penalties for Schedule A violations, including up to 20 years in prison for first-time offenses and up to 30 years for those with prior drug convictions, with mandatory minimum supervised release periods of three to six years depending on criminal history. The legislation also requires proper labeling of Schedule A substances using international chemical nomenclature and establishes registration requirements for those importing or exporting these substances for legitimate research, analytical, or industrial purposes. Researchers already studying substances when they are added to Schedule A are allowed to continue their work under specified conditions while they obtain the necessary permits.