The anti-receptor activator of nuclear factor kappa-B ligand (RANKL) antibody, Denosumab (DEN), was approved in April 2012 in Japan, but a Dear Healthcare Professional Letter of Rapid Safety Communication was released in September, 2012 by the regulatory authority because of the severe hypocalcemia risks. Currently, the effectiveness of this regulatory action has not been evaluated and, therefore, this study aimed to assess its impact on DEN-induced hypocalcemia using the Japanese Adverse Drug Event Report database (JADER). The case reports from April 2012 to September 2014 were collected from the JADER, which included 151642 adverse events for the primary suspected drugs. The reporting odds ratio (ROR) of hypocalcemia as a signal of the target adverse event was analyzed for DEN and zoledronic acid (ZOL, a reference drug). Changes in RORs were compared between the pre- (Pre, April 2012 to September 2012) and post- (Post 1, October 2012 to September 2013 and Post 2, October 2013 to September 2014) periods of the regulatory action. A decrease in the hypocalcemia ROR was observed for DEN in the post-periods, especially Post 2. Multivariate logistic regression analysis showed a significant decrease in hypocalcemia signal in Post 1 (p=0.0306 vs. Pre) and Post 2 (p=0.0054 vs. Pre). ZOL caused no significant changes in ROR of hypocalcemia, and none of the drugs caused ROR changes in jaw osteonecrosis (a reference adverse event). This study suggests that the regulatory action against hypocalcemia in DEN effectively decreased hypocalcemia signal. Further studies using medical information databases are needed to confirm this result.