Bisphosphonates are antiosteoporotic agents prescribed for patients with osteoporosis. Drug package inserts for bisphosphonate supplements indicate that their bioavailability is reduced by high levels of metal cations (Ca 2 , Mg 2 , etc.). However, standards for these cations in water used for taking risedronate have not been defined. Here, we examined the effect of calcium and magnesium in mineral waters on the bioavailability of the third-generation bisphosphonate, risedronate, following oral administration in rats. As risedronate is unchanged and eliminated renally, risedronate absorption was estimated from the amount excreted in the urine. Risedronate was dissolved in mineral water samples and administered orally at 0.35 mg/ kg. Urine samples were collected for 24 h after dosing. Risedronate was extracted from urine using ion-pair solid-phase cartridges and quantified by HPLC with UV detection (262 nm). Cumulative recovery of risedronate was calculated from the amount excreted in the urine. Key words risedronate; mineral water; drug interaction; urine excretion; rat Osteoporosis is a systemic disease characterized by reduced bone mass and structural deterioration of bone tissue, leading to an increased risk of fracture. According to the 2011 Guidelines for the Treatment of Osteoporosis, the number of patients with osteoporosis in the rapidly aging population of Japan is increasing and is now estimated at about 13 million. 1)The occurrence of a fracture significantly impacts a person's daily activities, so pharmacotherapy for the prevention of osteoporosis is recommended for patients who are at high risk of bone fracture. A variety of drugs are available for treating osteoporosis, including calcium agents, active vitamin D 3 , raloxifene, and bisphosphonates (BPs). BPs exhibit particularly potent bone resorption inhibitory activity and are therefore the first-choice drug for treating osteoporosis. 1)Because they inhibit resorption and thus increase bone mineral density, BPs play an important role in treating postmenopausal osteoporosis. The resorption-inhibiting activity of risedronate, a third-generation BP, is reported to be approximately 3000 times that of etidronate, a first-generation BP.2) Recently, risedronate began to be marketed for once per month (75 mg), in addition to daily (2.5 mg) and weekly (17.5 mg) dosing.The variety of mineral waters sold and consumed in Japan has increased in recent years. Annual consumption of mineral water per person in Japan was 25.7 L in 2014 (approximately 3 times that of 15 years ago).3) The consumption of mineral waters in Japan is predicted to increase further in coming years. For orally administered BPs, the package inserts advise patients and health care workers that the bioavailability of the drug may be reduced when taken with mineral waters containing high levels of metal cations (Ca 2+ , Mg 2+, etc.), as a result of chelation (Ajinomoto Co., Inc., Actonel ® package insert [2013/2]). As the hardness of a mineral water depends on its calcium and magnesium conten...