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.
Tablets that consisted of various composition ratios of magnesium oxide, 4 kinds of excipients, and 3 kinds of disintegrants were made by the direct powder compression method with a rotary tableting machine. These tablets were preserved for one week in an environment of 40℃ and 75% relative humidity, and hardness was measured. Hardness decreased about 40% in the tablet in which magnesium oxide had not been mixed compared with the hardness before preservation. On the other hand, the hardness of tablet before preservation was maintained, and the decrease in hardness was small in the tablet in which the magnesium oxide was mixed. Moreover, the larger the blending quantity of magnesium oxide was, the smaller was the level of the hardness loss. It was suggested that this was because a part of the prescribed powder dissolved into the moisture absorbed by magnesium oxide during preservation, this solution formed a liquid bridge between particles, this liquid bridge dried and became a solid bridge after the preservation, and the hardness of the tablet increased in the tablet in which the magnesium oxide was mixed.