Malnutrition may result in a phosphate-deficient state owing to a chronically insufficient phosphate intake. Concomitant iron deficiency is common and often supplemented by the intravenous route. It is not widely recognized that some parenteral iron formulations can induce hypophosphatemia. Herein we report a case of a severe and symptomatic hypophosphatemia (0.18 mM, normal range 0.8-1.4 mM) associated with an inappropriately reduced tubular reabsorption of phosphate (33%, norm >95%) in a malnourished patient with anorexia/bulimia who received 2 × 500 mg iron carboxymaltose (FCM) intravenously. Despite intravenous and oral phosphate supplements, it required 2 months to achieve a normal serum phosphate level. Our case demonstrates that in a chronically malnourished and phosphate-deficient state intravenous FCM could potentially be dangerous. If this form of iron application cannot be avoided, phosphate supplementation before and after iron infusion as well as close monitoring of phosphate levels are needed.