S-Butyrylthiocholine iodide is primarily used in biochemical research as a substrate for cholinesterase enzymes. It plays a critical role in assays designed to measure the activity of these enzymes, particularly butyrylcholinesterase. By serving as a synthetic analog of acetylcholine, it helps researchers study enzyme kinetics, inhibition, and the effects of various compounds on cholinesterase activity. This chemical is also utilized in diagnostic applications to detect and monitor conditions related to cholinesterase dysfunction, such as pesticide poisoning or certain neurological disorders. Its specificity and reactivity make it a valuable tool in both academic and clinical settings.