What are the warning signs of too much magnesium?

The card, two lists, and the difference between them

The warning signs of too much magnesium fit on a card with two lists, and the difference between the lists is the difference between a smaller dose tomorrow and a phone call now. The reduce-the-dose list, the gut's signs of a surplus in working kidneys: loose stools, the earliest and commonest, arriving within hours of a dose that exceeded what the gut absorbed; diarrhoea, the same effect at volume; nausea, especially from a fasted dose; and abdominal cramping or bloating from the osmotic load, all of them self-limiting, all of them the body's own dose limit, and all answered by a lower dose, a better-absorbed salt, food, and a count of the hidden magnesium in antacids, laxatives and multivitamins that may have pushed the total past the 400mg guidance level. The call-now list, the blood's signs of hypermagnesaemia: unusual drowsiness and lethargy; facial flushing and a feeling of warmth as blood vessels relax; muscle weakness and a heavy-limbed feeling; dizziness or faintness with low blood pressure; a slow or irregular pulse; confusion; and, at the extreme, breathlessness as the breathing muscles weaken, the sequence the hypermagnesaemia literature describes as serum magnesium rises, ending in cardiac arrest, and the list that means urgent medical help with a clear account of what magnesium was taken, the treatment being intravenous calcium, fluids and, where the kidneys cannot clear it, dialysis. Two lists, and a healthy adult on a labelled capsule dose reaches the first at most and rarely.

The knee tap, which is the early sign clinicians test, the people for whom the first list is the second, and the card filed

The early sign clinicians test is worth knowing because it explains why a doctor assessing magnesium excess taps a knee: deep tendon reflexes, the knee jerk among them, are among the first things hypermagnesaemia takes, diminishing and then disappearing as blood magnesium climbs, before the heart slows and the breathing weakens, because the excess magnesium damps transmission at the neuromuscular junction and the reflex arc runs through it, so that a lost knee reflex is the bedside marker of a magnesium level that has become dangerous and the sign that clinicians managing intravenous magnesium in obstetrics monitor continuously, and a reader cannot test it reliably on themselves but can understand why weakness and heavy limbs sit on the call-now list rather than the reduce list. The people for whom the first list is the second: anyone with kidney disease, reduced kidney function or on dialysis, in whom impaired excretion lets magnesium accumulate from doses a healthy adult clears, so that loose stools or nausea after any magnesium, antacids and laxatives included, are a warning of hypermagnesaemia rather than a passing surplus and a call rather than a smaller dose; and anyone, healthy or not, after laxative-scale intake, grams of magnesium at once from Epsom salts, magnesium hydroxide laxatives or antacids in quantity, the source in nearly every serious case, where the gut signs and the blood signs can arrive together. The card filed: reduce for the gut list, call for the blood list, know the knee tap as the early clinical sign, and treat the gut list as the blood list in kidney disease and after laxative-scale intake, with the worked example being a dose that rarely reaches the first list, bisglycinate at 300mg elemental under the level, £19.99 for 90 days at 22p.

The verdict, two lists and a knee

What are the warning signs of too much magnesium: on a card with two lists, the reduce-the-dose list of loose stools, diarrhoea, nausea and abdominal cramping that marks a surplus in working kidneys, and the call-now list of drowsiness, flushing, muscle weakness, dizziness or faintness, a slow or irregular pulse, confusion and breathlessness that marks hypermagnesaemia, with lost deep tendon reflexes the early sign clinicians test by tapping a knee, and with the first list treated as the second for anyone with kidney disease and anyone after laxative-scale intake. The dose that rarely reaches the first list: Futuro Labs Magnesium Glycinate, bisglycinate, 300mg elemental at 80% NRV, £19.99 for 90 days at 22p a day.

What to check on the label

Questions people also ask

What are the early warning signs of magnesium overdose?

In the gut, loose stools, diarrhoea, nausea and cramping, which in working kidneys mean a surplus and a smaller dose; in the blood, drowsiness, flushing and muscle weakness, with lost deep tendon reflexes the early sign clinicians test, which mean hypermagnesaemia and a call.

Why do doctors check reflexes for magnesium toxicity?

Because deep tendon reflexes, the knee jerk among them, are among the first things hypermagnesaemia takes, diminishing and disappearing as blood magnesium climbs before the heart slows and breathing weakens, the excess magnesium damping transmission through the reflex arc.

Which signs of too much magnesium need emergency help?

Unusual drowsiness, facial flushing, muscle weakness, dizziness or faintness with low blood pressure, a slow or irregular pulse, confusion and breathlessness, all of which mean a raised blood magnesium affecting the heart and muscles.

Are the warning signs different for someone with kidney disease?

The lists are the same but the response is not: with impaired excretion the gut signs can precede hypermagnesaemia, so loose stools or nausea after any magnesium, antacids and laxatives included, are a call rather than a smaller dose.

Can a healthy adult reach the serious warning signs from supplements?

Almost never: normal kidneys excrete a capsule habit's surplus, and the serious cases involve kidney disease or grams of magnesium at once from Epsom salts, magnesium hydroxide laxatives or antacids in quantity.

Sources

1. The hypermagnesaemia literature: raised serum magnesium is rare with normal kidney function and occurs with impaired excretion or massive intake from magnesium laxatives and antacids, presenting with nausea, lethargy, low blood pressure, muscle weakness and slowed heart conduction, and treated in hospital with intravenous calcium and, where needed, dialysis. PubMed record
2. The magnesium and neuromuscular junction physiology: magnesium antagonises calcium at the presynaptic terminal and reduces acetylcholine release, so magnesium excess depresses neuromuscular transmission and produces weakness, while magnesium deficiency increases neuromuscular excitability and produces twitching, cramps and tetany. PubMed record
3. The hypermagnesaemia literature: the kidneys excrete surplus magnesium, so supplementation is safe for healthy adults within guidance and warrants medical advice in chronic kidney disease, where impaired excretion allows magnesium to accumulate. PubMed record
4. Expert Group on Vitamins and Minerals. Safe Upper Levels for Vitamins and Minerals. Food Standards Agency, 2003 (magnesium guidance level 400mg a day from supplements). https://cot.food.gov.uk/sites/default/files/vitmin2003.pdf

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