Taking too much magnesium usually causes digestive upset at first, but in higher amounts it can dangerously slow your heart, blood pressure, and breathing, and can be life‑threatening in severe cases.

Quick Scoop

  • Early effects are mostly gut‑related : diarrhea, nausea, vomiting, stomach cramps.
  • As blood levels rise, people can feel very tired, confused, flushed, dizzy, with headaches and muscle weakness.
  • Severe overdose (usually with kidney problems or big medical doses) can lead to very low blood pressure, slow heartbeat, breathing trouble, paralysis, coma, and even cardiac arrest.
  • From food alone this is rare; it almost always involves supplements, laxatives/antacids, or IV magnesium, especially if the kidneys don’t work well.
  • If someone has trouble breathing, chest pain, extreme weakness, or feels like passing out after a big magnesium dose, this is an emergency and they need urgent medical care.

What “Too Much” Magnesium Does In Your Body

Magnesium helps control nerve signals, muscle contraction, and how your heart and blood vessels work. When levels get too high (hypermagnesemia), that calming effect turns into suppression: nerves, muscles, and the heart can all slow down too much.

Typical progression:

  1. Mild excess (common with supplements)
    • Diarrhea and loose stools (especially with magnesium oxide, citrate, hydroxide).
 * Nausea, stomach cramps, sometimes vomiting.
  1. Moderate excess (blood magnesium clearly elevated)
    • Lethargy, feeling very tired or “out of it.”
 * Facial flushing, dizziness, blurred vision, headache.

 * Muscle weakness and reduced reflexes.

 * Low blood pressure and slower heart rate.
  1. Severe overdose (medical emergency)
    • Very low blood pressure, dangerous heart rhythm problems, or cardiac arrest.
 * Severe muscle weakness or flaccid paralysis, loss of tendon reflexes.

 * Breathing difficulty, apnea (pauses in breathing), coma, and death if untreated.

A real‑world example in the medical literature describes older patients given magnesium oxide as a laxative who developed severe hypermagnesemia with shock and coma, especially when they also had constipation and kidney impairment.

How People Actually Get Too Much

Hypermagnesemia is uncommon in healthy people because normal kidneys quickly get rid of extra magnesium. When overdose does happen, it usually fits into one of these scenarios:

  • High‑dose supplements
    • Taking well above the labeled dose, or stacking multiple products (sleep aids, “calming” powders, multivitamins) that all contain magnesium.
* The current tolerable upper intake level for _supplemental_ magnesium in adults is 350 mg per day, mainly to avoid diarrhea, though some experts are now reevaluating this limit.
  • Laxatives and antacids with magnesium
    • Products like magnesium hydroxide (milk of magnesia) or some antacids can deliver grams of magnesium per day if overused.
* Poison‑center data highlight that “too much magnesium” calls often involve these over‑the‑counter bowel products.
  • IV magnesium in hospitals
    • Medical errors (wrong dose, wrong rate, confusion about units) can cause sudden, severe overdose affecting the heart and nervous system.
  • Underlying kidney problems
    • Chronic kidney disease, acute kidney failure, or older age with reduced kidney reserve make it much harder to clear magnesium, so even regular doses can build up.

When To Worry And What To Do

Mild, short‑term “took a bit too much”

If someone accidentally takes an extra standard dose and only has:

  • Loose stools
  • Mild stomach cramps
  • Slight nausea

they can usually:

  • Stop magnesium supplements and magnesium‑containing laxatives/antacids for a day or more.
  • Drink fluids (if not on fluid restriction) to help the kidneys clear it.
  • Watch for worsening symptoms like extreme weakness, confusion, or chest symptoms.

Red‑flag symptoms (seek urgent care)

Go to urgent care or emergency services if, after taking magnesium:

  • You feel extremely weak, can’t stand or move normally, or lose reflexes.
  • You have chest pain, pounding or very slow heartbeat, or feel like you might pass out.
  • You get serious shortness of breath, trouble catching your breath, or any signs of respiratory distress.
  • There is confusion, very unusual drowsiness, or difficulty waking up.

In hospital, treatment can include IV calcium to protect the heart, IV fluids and diuretics to flush magnesium out, and dialysis if the kidneys cannot clear it.

Safety Tips If You Use Magnesium

  • Check all supplement labels so you know your total daily magnesium from pills, powders, “sleep drinks,” and antacids.
  • If you have kidney disease, heart rhythm problems, or take medications that affect magnesium or blood pressure, ask your clinician before starting or increasing magnesium.
  • Start low and increase slowly if you’re using magnesium for constipation or sleep, and stop or reduce the dose if you get diarrhea.
  • Never exceed the package’s maximum dose without medical guidance, especially with laxatives and antacids.

Bottom line (TL;DR)

  • Most people who take a bit too much magnesium get diarrhea and stomach upset, then improve after stopping it.
  • Very high doses or use in people with kidney or heart issues can cause serious, sometimes fatal problems with blood pressure, heart rhythm, muscles, and breathing.
  • If there are severe symptoms after a large dose, this is an emergency; seek immediate medical help.

Information gathered from public forums or data available on the internet and portrayed here.