要旨 　硫酸マグネシウム（以下Mg）静注療法は，破傷風の筋痙攣・強直や自律神経障害への有効性が報告されている。しかし，その投与量の調整方法は確立されていない。本邦で破傷風患者は稀であるが，我々は異なる指標で硫酸Mgの投与量を調整した2名の破傷風患者を経験した。患者1では臨床所見にもとづいて調整したところ，筋痙攣・強直や自律神経障害のコントロールは良好であったものの重篤な高Mg血症を生じ，経過中にQT延長と気管内出血の合併症を来した。患者2では血清Mg濃度で投与量を調整したところ，患者1よりも血圧変動が目立ったものの，有害事象を生じることなく良好な転帰を得ることができた。破傷風に対して硫酸Mg静注を行う際には，ある程度の自律神経障害が残存したとしても，重篤な高Mg血症を来さないように血中濃度をもとに投与量を調整するほうが好ましい可能性が示唆された。 ABSTRACT Continuous intravenous magnesium (Mg) infusion is reported to be effective for improving muscle spasm, myotonia, and autonomic neuropathy, all of which pose problems in the systemic management of tetanus. However, much remains unknown regarding adverse events such as arrhythmia secondary to hypermagnesemia, and few studies have defined the ideal serum concentration of Mg. Through our experience with managing two cases of tetanus, which is a rare condition in Japan, we have examined the efficacy and safety of Mg. In Case 1, we managed serum Mg concentration based on clinical findings. Although we achieved favorable control of muscle spasm, myotonia, and autonomic neuropathy, the patient had a high serum concentration of Mg and showed QT prolongation and endotracheal bleeding secondary to hypermagnesemia as adverse events. In Case 2, we maintained the patient’s serum Mg concentration at 4.5−7.5mg/dL. The patient demonstrated greater variations in vital signs than in Case 1 but did not develop any adverse events associated with hypermagnesemia. Our experience suggests that, administering Mg until it completely suppresses muscle spasm, myotonia, and autonomic neuropathy may cause serious side effects due to hypermagemia, patients with tetanus can be managed more safely when serum concentrations of Mg are maintained within an appropriate range (4.5−7.5mg/dL).