Succinylcholine acts as a depolarizing muscle relaxant. It imitates the action of acetylcholine at the neuromuscular junction, but is not as readily degraded by acetylcholine esterase. The prolonged stimulation of the acetylcholine receptor results first in disorganized muscle contractions, then in profound relaxation.
Its medical uses are limited to short-term muscle relaxation in anesthesia and intensive care, usually for facilitation of tracheal intubation. Despite its many undesired effects on the circulatory system and skeletal muscles (including malignant hyperthermia[?], a rare but life-threatening disease), it is still much used because it arguably has the fastest onset of action of all muscle relaxants.
A single intravenous dose of 0.6 to 1.0 milligram per kilogram of body weight will cause flaccid paralysis within a minute of injection. For intramuscular injection higher doses are used and the effects lasts somewhat longer. Succinylcholine is quickly degraded by plasma cholinesterase[?] and the duration of effect is usually in the range of a few minutes. When plasma levels of cholinesterase are greatly diminished or an atypical form of cholinesterase is present (an otherwise harmless inherited disorder), paralysis may last much longer.
Side effects include muscle fasciculations, acute rhabdomyolysis with hyperkalemia[?], changes in cardiac rhythm including bradycardia, cardiac arrest and ventricular dysrhythmias[?]. In children with unrecognized neuromuscular diseases, a single injection of succinylcholine can lead to massive release of potassium from skeletal muscles with cardiac arrest.
The ability to paralyse the respiratory muscles have led to its use as part of a lethal injection.