Magnesium glycinate: side effects and interactions

The most common side effect is in the gut

The side effects people notice most with magnesium supplements are diarrhoea, nausea and abdominal cramping.

The reason is simple. Magnesium that is not absorbed stays in the bowel and draws water in with it. That is the same mechanism that makes some magnesium compounds effective laxatives, and it is why these effects become more likely as the dose goes up.

The forms most often linked with diarrhoea are magnesium carbonate, chloride, gluconate and oxide. That does not mean other forms never cause it. It means these are the ones most commonly reported.

This is also the reason for the upper level of 350 mg a day for magnesium from supplements and medicines. The limit is not about magnesium building up in the body in healthy people. It is set to avoid the gut effects of a large dose arriving at once, which is why it does not apply to magnesium from food.

The 350 mg figure is elemental magnesium, not the weight of the compound on the front of the pack. Our guide to elemental magnesium explains how to find the right number on an Australian label.

What the research shows for magnesium glycinate

Magnesium glycinate has a good tolerability record in the trials that have used it.

In a 1994 stable-isotope trial comparing magnesium diglycinate with magnesium oxide in patients who had undergone bowel surgery, the glycinate was better tolerated by every patient. That was a group in whom magnesium supplements commonly cause diarrhoea at useful doses.

In a 2025 placebo-controlled trial, 155 adults took either 250 mg of elemental magnesium as bisglycinate or a placebo, daily for four weeks. There were no serious adverse events. 93% of participants reported no adverse events at all, and side effects were reported less often in the magnesium group than in the placebo group. The two people who withdrew because of stomach problems were both taking the placebo.

Those results are reassuring, and they have limits worth stating plainly.

• They are short. Four weeks does not tell you what happens over a year of daily use.
• They are small. A trial of 155 people can miss side effects that are uncommon.
• The participants were selected. The 2025 trial enrolled healthy adults aged 18 to 65 and excluded people with kidney or liver insufficiency, people who were pregnant or breastfeeding, and people using laxatives. Its results do not describe what happens in those groups.

So the fair summary is that magnesium glycinate has been well tolerated at around 250 to 300 mg of elemental magnesium a day in healthy adults over a few weeks. It is not evidence that it cannot cause gut symptoms, and it says nothing about people with reduced kidney function.

Too much magnesium in the blood

A high level of magnesium in the blood is called hypermagnesaemia. In people with healthy kidneys it is rare, because the kidneys clear what the body does not need. That is also why you cannot get too much magnesium from food.

Toxicity has usually been reported with very large doses, typically more than 5,000 mg a day, from laxatives and antacids that contain magnesium. Symptoms tend to appear as blood levels rise and can progress from low blood pressure, nausea, vomiting and facial flushing to muscle weakness, difficulty breathing, an irregular heartbeat and, at the extreme, cardiac arrest.

The main risk factor is reduced kidney function. When the kidneys cannot clear magnesium properly, it can build up even at doses that would be unremarkable for someone else.

A study of older hospital patients in Japan shows how the risks combine. It looked at people prescribed magnesium oxide for constipation. High blood magnesium occurred in 11.6% of those taking 900 mg or more of magnesium oxide a day, in 27.0% of those with markedly reduced kidney function, and in 53.1% of those with both. Those are laxative doses of magnesium oxide in older people under hospital care, not typical supplement use. The pattern is still the point: dose and kidney function together matter far more than either alone.

Medicines that interact with magnesium

Magnesium can change how some medicines work, and some medicines change how much magnesium the body keeps.

Osteoporosis medicines (oral bisphosphonates). Magnesium can reduce how much of these medicines is absorbed. Take them at least two hours apart from magnesium.

Some antibiotics. Magnesium can bind to tetracycline antibiotics, such as doxycycline, and to quinolone antibiotics, such as ciprofloxacin, forming compounds the body cannot absorb well. Take these antibiotics at least two hours before, or four to six hours after, a magnesium supplement.

Thyroid hormone medicine (levothyroxine). In a 2025 crossover trial in 15 healthy adults, taking levothyroxine together with magnesium reduced how much of it was absorbed, by 12% with magnesium aspartate and by 7% with magnesium citrate. The researchers advised that people taking levothyroxine should take it separately from magnesium-containing products.

Diuretics. Loop and thiazide diuretics increase the amount of magnesium lost in urine and can lower magnesium levels over time. Potassium-sparing diuretics work the other way and reduce magnesium loss.

Reflux medicines (proton pump inhibitors). Long-term use, usually beyond a year, can lower magnesium levels.

Other medicines that can lower magnesium. healthdirect also lists some medicines for diabetes, some antibiotics and antiviral medicines, chemotherapy and immunosuppressants.

This is not a complete list. If you take any regular medicine, ask your pharmacist before adding magnesium. They can check your particular medicines and tell you how to time the doses.

Check what else is in the product

Many magnesium products in Australia are not magnesium alone. The other ingredients have their own safety considerations.

Vitamin B6. The TGA notes that vitamin B6 is found in a range of products, including magnesium supplements, multivitamins and zinc and iron supplements. Taking too much vitamin B6 can damage nerves, a condition called peripheral neuropathy, and the risk adds up when several products each contain some. The TGA advises that if you experience tingling, burning or numbness in your hands or feet, you should stop taking the product immediately and seek medical advice as soon as possible. From 1 June 2027, oral products with more than 50 mg of vitamin B6 per daily dose will only be available with a pharmacist's advice. Check every label you take, not just one.

Glycine. Magnesium glycinate carries a substantial amount of the amino acid glycine alongside the magnesium, around 1.5 g in a 250 mg magnesium dose. Our guide to what magnesium glycinate is explains why that matters when comparing forms.

Other magnesium. Antacids, laxatives and multivitamins can all contain magnesium. The elemental amounts across everything you take are what count towards the 350 mg upper level.

Who should get advice before starting

• People with reduced kidney function, including older people whose kidney function has declined with age.
• People taking prescription medicines, especially those listed above.
• People who are pregnant or breastfeeding. Speak with your doctor, midwife or pharmacist.
• Children. Upper levels for children are much lower than for adults: 65 mg a day from supplements for ages 1 to 3, and 110 mg for ages 4 to 8.
• Anyone already taking a magnesium-containing laxative or antacid.

When to stop and seek help

Stop taking the supplement and speak with a doctor or pharmacist if you have diarrhoea that does not settle, or if you develop tingling, burning or numbness in your hands or feet while taking a product that contains vitamin B6.

Muscle weakness, difficulty breathing, feeling faint or an irregular heartbeat need urgent medical attention. Call triple zero (000) in an emergency.

If you think someone has taken too much of a supplement, the Poisons Information Centre can be reached on 13 11 26, 24 hours a day, anywhere in Australia.

Nothing on this page is a substitute for individual medical advice.