Brief Answer:
there are various potential causes

Detailed Answer:
Hello,

hypomagnesemia is not a very common finding. It can be caused by various causes including situations where the patient does not receive adequate amounts of magnesium or loses increased amounts of magnesium in the gastrointestinal tract or the urine. Certain drugs may impair magnesium levels too.

- inability to eat (starvation for any cause, alcoholics who only drink alcohol and do not eat and patients receiving total parenteral nutrition)
- increased gastrointestinal losses: diarrhea is a potential cause particularly if protracted. Fistulas is another. Finally loss of stomach fluids is a potential cause (vomiting or nasogastric suction)
- certain drugs may decrease magnesium in the blood. Some commonly used agents include proton pump inhibitors, diuretics, some antibiotics, etc.
- renal causes: renal syndromes (Gitelman's, Bartter's, chronic metabolic acidosis, primary hyperaldosteronism, etc).

If you had diarrhea for a long time then this is the most likely cause. The cause of the diarrhea has to be investigated though starting with cultures and microscopic examination of the stool. Magnesium supplementation in the meantime is essential.

I hope it helps!
Contact me again if you require more information or clarifications.

Kind Regards!

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