How Much Is Too Much Magnesium?

How much is too much? All four doctors say that consuming over 400 milligrams of magnesium a day in supplement form is too much. As you may remember, the recommended daily requirement for magnesium is between 400 and 420 milligrams. But taking more than 400 milligrams a day is too much because, as the doctors explain, if your source of magnesium is coming from a supplement, the body is likely getting a big dose of it all at once, which can overwhelm the system. And it’s worth remembering that you’re still consuming at least some magnesium through your diet, even if you’re falling short of the recommended daily amount.

Dr. Benin says that it’s easier to reach this upper limit than you may think, especially if you’re taking several supplements. For example, magnesium could be in a multivitamin as well as a supplement you’re taking for sleep. “Some over-the-counter products can add up quickly if you’re not paying attention to the total daily intake,” he says.

Here’s the thing: You’re definitely going to know if you’re getting too much magnesium because all four doctors say you’ll experience diarrhea. If your magnesium supplement has you running to the toilet, you’re taking too much. If that happens, all the experts say to dial back your dose and consider splitting it up instead of taking the whole dose at the same time.

Other than diarrhea, Dr. Benin says that there are some more serious complications that can come from taking too much magnesium, particularly for those with impaired kidney function. “At very high levels, magnesium can become much more serious and may lead to weakness, low blood pressure, confusion, abnormal heart rhythms, difficulty breathing, and in severe cases, cardiac arrest,” he says.

Medications that magnesium can interfere with

As with any supplement, it’s worth a conversation with your doctor before starting a magnesium supplement if you have any underlying health conditions or are taking any medications. There are some medications that don’t play well with magnesium.

Dr. Anderson says that magnesium can reduce the absorption of several medications, including certain antibiotics, bisphosphonates used to treat osteoporosis, and levothyroxine used for hypothyroidism. “If you’re taking one of these medications, magnesium may need to be taken several hours apart, although the timing depends on the specific medication,” she says.

Dr. Benin adds to this, saying, “Many of my patients are often shocked to learn that a supplement they consider ‘harmless’ can actually affect how well a prescription medication is absorbed.”

There’s no doubt that many people can benefit from taking a magnesium supplement. But as with most things in life, it’s possible to overdo it. Especially if you’re taking multiple supplements, check how much magnesium is in each one that you’re taking. If the total is more than 400 milligrams, you’re taking too much, something your G.I. tract is sure to clue you in on.