Primidone is an anticonvulsant drug taken for the treatment of essential tremor. Side effects may become more pronounced with increasing doses. However, atrial fibrillation as a side effect of the drug has neither been reported in literature nor is it specifically indicated by the package insert. Here we would like to report a patient who developed sedation, confusion, and atrial fibrillation after primidone use. A 68-year-old woman presented to our emergency department with nausea, vomiting, perspiration, and somnolence some time after taking primidone (Mysoline) 250 mg. It was learnt that Mysoline 62.5 mg (a quarter of the 250 mg tablet) had been prescribed by a neurologist for treating essential tremor 2–3 weeks before, although the patient had inadvertently taken a tablet per day. An admission electrocardiography (ECG) at the emergency department showed atrial fibrillation with a rapid ventricular response. The cardiology and neurology departments agreed that the atrial fibrillation episode was a side effect of primidone. It is recommended to instruct patients on and warn them against the side effects when primidone is taken for the first time. It should be kept in mind that in elderly patients presenting to emergency departments, drugs such as primidone, which may cause sedation, confusion, and atrial fibrillation, may be the cause.Primidone is an anticonvulsant drug taken for the treatment of essential tremor. Side effects may become more pronounced with increasing doses. However, atrial fibrillation as a side effect of the drug has neither been reported in literature nor is it specifically indicated by the package insert. Here we would like to report a patient who developed sedation, confusion, and atrial fibrillation after primidone use. A 68-year-old woman presented to our emergency department with nausea, vomiting, perspiration, and somnolence some time after taking primidone (Mysoline) 250 mg. It was learnt that Mysoline 62.5 mg (a quarter of the 250 mg tablet) had been prescribed by a neurologist for treating essential tremor 2–3 weeks before, although the patient had inadvertently taken a tablet per day. An admission electrocardiography (ECG) at the emergency department showed atrial fibrillation with a rapid ventricular response. The cardiology and neurology departments agreed that the atrial fibrillation episode was a side effect of primidone. It is recommended to instruct patients on and warn them against the side effects when primidone is taken for the first time. It should be kept in mind that in elderly patients presenting to emergency departments, drugs such as primidone, which may cause sedation, confusion, and atrial fibrillation, may be the cause.

Keywords: Primidone, atrial fibrillation, sedation, emergency department