Does magnesium interact with any medications?

Magnesium-rich supplements and medicines reduce absorption of oral bisphosphonates taken for osteoporosis.

Magnesium binds tetracycline and quinolone antibiotics into insoluble complexes, which is why they should not be swallowed together.

Long-term treatment with loop or thiazide diuretics increases magnesium loss in urine and can deplete it.

Proton pump inhibitors taken for more than about a year can drive blood magnesium below normal.

What this means for you

Magnesium has more real drug interactions than most nutrients, and most of them are solved by putting a few hours between two tablets rather than by giving anything up.

Two are about absorption. Magnesium forms insoluble complexes with tetracycline antibiotics such as doxycycline and with quinolones such as ciprofloxacin, which means neither gets in properly if they are swallowed together. And magnesium-rich supplements or medicines reduce absorption of oral bisphosphonates such as alendronate, which people take for osteoporosis. In both cases the fix is timing.

Two run the other way, where the drug depletes you.

Not for everyone. Long-term loop or thiazide diuretics increase magnesium losses in urine and can deplete it. And prescription proton pump inhibitors taken for more than about a year can drive blood magnesium below normal, which is worth knowing because those drugs are often taken for years without review.

The proton pump inhibitor effect has a size now. Pooling the observational studies, low blood magnesium turned up in 19.4 per cent of people taking one against 13.5 per cent of people not, which works out as an odds ratio of 1.71. A high dose roughly doubles the odds again against a low one. Most people on these drugs never notice a thing. The point of knowing the number is to have magnesium on the list when a prescription that has been running for years finally comes up for review.

One more caution belongs here even though it is not an interaction. If your kidney function is reduced, magnesium accumulates instead of clearing, and supplements then need medical supervision. That includes the magnesium hiding in antacids and laxatives, which people rarely count as supplements at all. A trial put numbers on that risk. In people with chronic kidney disease, a year of magnesium hydroxide caused gut trouble in 35 of them against nine on placebo, and there were five deaths and six cardiovascular events on magnesium against two deaths and none on placebo. The calcification it was meant to slow did not move.

If magnesium is on your prescription list rather than in your cupboard, the form is worth asking about. Gluconate and chloride cause diarrhoea less often than the other salts. Carbonate is too insoluble to be absorbed properly, and citrate dissolves easily enough to loosen stools more than oxide does. A prescriber choosing between them is choosing how much of your week the tablet takes.

Worth remembering that the upper limit applies only to supplements and medicines. Magnesium in food and drink has never needed one. That 350 mg ceiling came from the lowest dose at which an adverse effect was seen, 360 mg, divided by an uncertainty factor of 1.0. The adverse effect was diarrhoea, and most of the studies it rests on never defined what they counted as diarrhoea. So the ceiling marks the point where supplements start upsetting people rather than the point where they start doing harm.

If you take magnesium for mood rather than for a medical reason, know what the evidence is. Seven randomised trials in 325 adults with a depressive disorder found lower depression scores, and the same review asks for larger trials before that becomes advice. It is not a substitute for treatment, and it does not interact with antidepressants in any documented way.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Full guide: Magnesium: widely under-eaten, and widely over-promised