Non-prescribed antacid drugs that contain magnesium and aluminum are widely used in the treatment of gastritis and peptic ulcer. One of the side effects of these antacid drugs is that they bind phosphate in the gut and result in its malabsorption. In this paper, a 42-year-old female patient who used magnesium hydroxide (Magnesie calcinee powder 100 g) to benefit from its laxative feature, and developed osteomalacia after losing 90 kg in 2 years will be presented by going through the related literature. She had widespread joint pain and could hardly walk without the help. Ca, P and vitamin D were at lower limit of normal, ALP, Mg and PTH were increased in her laboratory tests. There were stress fractures at the femur neck and at the upper part of the tibia in plane radiographies. The patient was hospitalized with the diagnosis of osteomalacia and she was treated successfully.