The SIMSA Act of 2024 amends the Controlled Substances Act to establish a new Schedule A for controlled substance analogues that are imported or offered for import into the United States. Schedule A substances are defined as drugs with chemical structures substantially similar to existing controlled substances and with similar or greater stimulant, depressant, or hallucinogenic effects. The Attorney General may temporarily schedule substances to Schedule A for up to 5 years (with possible 180-day extensions) and then permanently schedule them after at least 3 years, subject to HHS review and approval. The bill imposes criminal penalties for importing or manufacturing Schedule A substances, including up to 20 years imprisonment (or life if death or serious bodily injury results), with enhanced penalties for repeat offenders, and requires proper IUPAC nomenclature labeling for imported Schedule A substances.