Magnesium oxide tablets are commonly used as antacids or laxatives. They are mainly formulated for the elderly to relieve constipation. Although these tablets normally undergo rapid disintegration, moisture absorption causes an increase of the disintegration time. However, magnesium oxide tablets are sometimes dispensed in a one-dose package. In this study, we examined the effects of moisture absorption of a magnesium oxide tablet stored in rotor cassettes on formulation stability. The tablets were stored in rotor cassettes under 55％RH or 75％RH conditions for 7 days. These tablets were analyzed for weight, hardness, disintegration time, dissolution test, friability, and breakage after storage in rotor cassettes. Although the weight increased in a humidity-dependent manner, moisture absorption of the tablets significantly decreased when a desiccating agent was used. In contrast, the other tablet properties showed little difference. We further examined whether moisture absorption of the tablets stored in rotor cassettes caused deterioration in quality when stored in a one-dose package under the condition of 75％RH. Our results show that moisture absorption of the tablets enhanced hardness and delayed disintegration time. This delay in disintegration time of magnesium oxide tablets increases the risk of choking on a tablet and reduces the compliance of patients who have difficulty swallowing tablets. In conclusion, we demonstrated that moisture absorption affects the quality of magnesium oxide tablets while the preservation after one-dose packaging. It is important for pharmacists to carefully consider the effects of moisture absorption of hygroscopic tablets stored in rotor cassettes, when tablet packaging machines are used.

酸化マグネシウム錠は速崩壊性の錠剤であるが，吸湿により崩壊遅延を引き起こす．その理由として，酸化マグネシウムが吸湿時に粉体粒子間の固体架橋を引き起こすことが原因である可能性が考えられている．また，酸化マグネシウムのみで作製した錠剤では，吸湿により錠剤表面の構造変化が生じることが明らかにされている．しかし，上市されている酸化マグネシウム錠には様々な添加剤が含まれており，それらの錠剤での吸湿による表面構造の変化はこれまで報告されていない．本研究では，酸化マグネシウム錠の一つであるマグミット^® 錠の表面にはマイクロスケールでの凹凸構造が存在していることを見いだした．錠剤表面の凹凸構造は，水との接触面積を増大させ，素早い崩壊性に関与していると考えられたが，この構造は吸湿による崩壊遅延を引き起こした場合においても変化しないことが明らかになった．

Magnesium oxide tablets are sometimes crushed prior to administration to patients having difficulties in swallowing tablets. However, the quality of magnesium oxide formulations may decrease when stored for a long time as a one-dose package, owing to the absorption of high amounts of moisture by magnesium oxide. Additionally, as magnesium oxide solution is alkaline, it can interact with other medicines, making it incompatible for one-dose packaging. There are three types of magnesium oxide powder formulations: crushed magnesium oxide tablets, magnesium oxide bulk powder, and magnesium oxide granules. However, the appropriate formulation of magnesium oxide for one-dose packages is unknown. The stability of powder formulations stored as one-dose packages at 75％ RH for 3 months was evaluated by analyzing the change in weight change and dissolution. The dissolution of the three powder formulations did not change for 3 months. Although the weight had increased in all the formulations, the change in the weight of magnesium oxide granules was the lowest among the three formulations. Isoniazid and levodopa tablets have been reported to interact with magnesium oxide tablets. The incompatibility between the crushed powder of these medicines and magnesium oxide powder formulations was analyzed when stored as one-dose packages. Among the three formulations, incompatibility was the lowest for granule. In conclusion, preserving magnesium oxide and other formulations that causes incompatibility should basically be avoided. However, magnesium oxide granules can be used for storing with the other formulations that interact with magnesium oxide within a term to circumvent incompatibility.