Introduction: The drug metabolising enzyme pseudocholinesterase (butyrylcholinesterase) is responsible for the breakdown of the muscle relaxant medicines mivacurium and succinylcholine. Following infusion of mivacurium and succinylcholine, a deficiency of any kind might produce protracted paralysis and apnoea. Case presentation: It's a case study of a patient who developed long-term apnea after taking mivacurium. Conclusion: Prolonged blocks may be encountered due to mivacurium use. The diagnosis of pseudocholinesterase enzyme deficiency can be given after a careful clinic supervision and peripheral nerve stimulator monitoring. A decrease in the activity of pseudocholinesterase enzyme and improvement in neuromuscular function will help verifying our diagnosis. Instead of pharmacological applications that may further complicate the situation, what should be done in such patients is to wait until the block-effect goes down by the help of sedation and mechanical ventilation.