(pharmacology) A muscle relaxant administered intravenously in the form of its besylate C₆₅H₈₂N₂O₁₈S₂ especially in conjunction with general anesthesia to facilitate insertion of an endotracheal tube or to relax skeletal muscles during surgery or mechanical ventilation. It counteracts the action of the neurotransmitter acetylcholine at neuromuscular junctions by binding to the same receptors it acts upon.