1-Methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTHIQ, CAS 4965-09-7) is an organic compound featuring a tetrahydroisoquinoline core structure with a methyl group at the 1-position. It is a colorless to pale yellow liquid at room temperature and serves as a versatile nitrogen-containing heterocyclic building block. This compound is primarily utilized as a key intermediate in pharmaceutical research and synthesis. Its tetrahydroisoquinoline scaffold is a privileged structure found in numerous bioactive molecules and active pharmaceutical ingredients (APIs), particularly those targeting the central nervous system. It is a crucial precursor in the synthesis of various alkaloids and potential therapeutic agents. Due to its structural features, 1-MeTHIQ is also employed as a base or ligand in certain organic transformations and is a common starting material in medicinal chemistry for constructing more complex heterocyclic systems. It should be handled with care in a well-ventilated area, using appropriate personal protective equipment.