What it is
Magnesium looks harmless and so few questions are asked. At the counter it matters from two sides: what it changes in the absorption of other medicines, and what other medicines do to the magnesium in the body.
Many everyday medicines lower it: proton pump inhibitors taken for months (medicines agencies have issued a warning about hypomagnesaemia), loop and thiazide diuretics, sustained alcohol. Conversely, magnesium salts bind other drugs in the gut and they are absorbed less.
There is no health claim that can be applied to correct the effects of a medicine. If you suspect a deficit caused by medication, refer for a blood test.
The four clashes to know
What crosses most often in pharmacy:
- SPACE FROM DRUGS THAT BIND: quinolones and tetracyclines (2 h before or 4-6 h after per the data sheet), bisphosphonates (30 min at least, better another time of day), levothyroxine (4 h), iron.
- WATCH MAGNESIUM WITH LOSSES: omeprazole and similar in prolonged use, diuretics, alcohol, chronic diarrhoea.
- AVOID ACCUMULATION: kidney failure, older adults with low filtration; also if they take magnesium antacids daily.
- ADD UP THE TOTAL: an antacid, a laxative and a magnesium supplement can add together without the customer having thought of it.
At the counter
When to recommend it
- Space magnesium from the antibiotic (quinolone, tetracycline) and from levothyroxine
- Always ask about omeprazole, diuretics and alcohol before advising
- Divide the dose and take it with food if it causes diarrhoea
When not to
- With moderate or severe kidney failure without their doctor knowing
- At the same time as quinolones, tetracyclines, bisphosphonates or levothyroxine
- As a "protector" against omeprazole without the doctor assessing the blood test
- With heart block or marked bradycardia without checking
Warnings
- Important QUINOLONES AND TETRACYCLINES: magnesium markedly reduces their absorption; space per data sheet and mention it when dispensing the antibiotic.
- Important LEVOTHYROXINE: space 4 hours; if taken together the effective dose falls and TSH may rise.
- Important BISPHOSPHONATES: take magnesium at another time of day; absorption of the bisphosphonate falls.
- Important KIDNEY FAILURE: magnesium accumulates; ask and refer. It is not a harmless mineral in this case.
- Caution OMEPRAZOLE AND SIMILAR IN PROLONGED USE, DIURETICS AND ALCOHOL: they can lower magnesium. Symptoms (cramps, palpitations, tiredness) are referred for a blood test; they are not masked with a supplement.
- Caution HIGH DOSES: diarrhoea, nausea and abdominal pain. Divide the dose or change salt; above 250 mg of elemental magnesium a day, only with medical indication.
- Worth knowing An antacid with magnesium hydroxide already supplies magnesium: add it to the supplement.
Pharmacokinetics
- Absorption
- Absorption falls when a lot is taken at once and with other minerals or drugs that chelate it.
- Distribution
- Bone and muscle; the reserve is large and slow.
- Metabolism
- None.
- Elimination
- Renal. With low filtration it accumulates; with diarrhoea or diuretics it is lost.
Who needs an extra warning
| Aspect | Spacing warning | Referral warning |
|---|---|---|
| Antibiotics | Quinolones and tetracyclines | If sustained diarrhoea |
| Thyroid | Levothyroxine: 4 hours | If palpitations |
| Bone | Bisphosphonates: another time of day | If chest pain on swallowing |
| Kidney | Not applicable | Always before high doses |
| Gastric protection | Not applicable | If omeprazole for more than a few months |
Space if the problem is absorption; refer if the problem is loss or the kidney.
Self-assessment
Three questions. When you check your answers you will see the explanation for each one.
-
Together it lowers levothyroxine absorption.
-
There is a warning about hypomagnesaemia with prolonged proton pump inhibitors.
-
Magnesium accumulates when the kidney does not filter well.
Training content. It does not replace the summary of product characteristics or clinical judgement.