Lithium (Li+) is prescribed to 1‰ of the French population (\ 60,000 patients). More than half of patients describe subjective symptoms linked to Li+ (SSL) similar to those of chronic fatigue syndrome: muscle weakness, fatigability, cognitive disorders, emotional blunting. SSL are the 2nd cause of stopping Li+ (28%), just behind kidney problems (30%). In animals, the administration of lithium (Li+) increases the urinary excretion of phosphate (Pi) by 6, inducing phosphate diabetes (DPi). However, idiopathic forms of DPi explain up to 10-15% of chronic fatigue syndromes and these disappear when supplementing with Pi (± vitamin D). In humans, the introduction of Li+ leads to a reduction in serum phosphate but we have not found any publication on the possible induction of DPi or on a possible link between DPi and SSL. However, the dosages necessary to detect a DPi are carried out during the annual follow-up assessment of patients on Li+. All you have to do is calculate the standardized maximum reabsorption rate of Pi (TmPi) to make the diagnosis! Finally, if the patients presenting with DPi turn out to be the same as those who complain of SSL, we can imagine that correcting the first by simple supplementation with Pi (± vitamin D) could provide relief.