Is Magnesium a Wonder Drug? FOAMcast Reviews the Studies

Fads come and go. However, in the world of free open access medical education (#FOAMed), some favorite topics have stood the test of time. For example, FOAMed loves to talk about ketamine, managing the difficult airway, and, for some reason, magnesium. People seem to love the idea that magnesium, a humble group 2 alkaline Earth metal, might possibly be an effective treatment for so many different medical conditions.

Recently on our podcast, we reviewed the evidence for using magnesium for rate control in atrial fibrillation, pain relief in migraine headaches, and symptom control for asthma exacerbations. Let’s take a look at some of the data we considered.

A recent randomized, double-blind, placebo-controlled trial conducted in Tunisia assessed the efficacy of intravenous magnesium sulfate as an adjunct for rate control in patients with rapid atrial fibrillation.¹ Patients who presented to the emergency department in rapid atrial fibrillation (rates greater than 120 beats per minute) received any rate control agent chosen by the treating physician. They were randomized to additionally receive either IV magnesium (either a low or a high dose) or a similar amount of IV fluid as placebo. The results? Magnesium seems to have helped achieve a heart rate of <90 beats per minute (or a 20 percent decrease from the initial rate) at four hours. Both the low and high doses of IV magnesium (4.5 g and 9 g) were effective, but the low dose was less likely to cause side effects such as flushing or bradycardia; hypotension was exceedingly rare.

However, there are some caveats to consider. Most of all, this trial does not reflect the traditional practice pattern in the United States. The most common rate control agent used in the trial was digoxin. That meant that the average time to rate control in the group that received digoxin alone (the placebo arm) was around 8.5 hours. Today, when using calcium channel blockers like diltiazem or beta blockers like metoprolol, we expect rate control far sooner. While some patients in this study did receive atrioventricular nodal blockers that more closely reflect common practice in the United States, the data for that subset were not shown in the paper.

Interestingly, the authors claim that the results in that subset also favored magnesium as an adjunct. We would call this a soft win for magnesium, but we would love to see a trial that mainly uses beta blockers or calcium channel blockers as opposed to digoxin.

Next, we looked over some evidence on the use of magnesium for migraine headaches. The highest-quality randomized, placebo-controlled trial we know about is a study that assessed 2 g magnesium IV as an adjunct to 20 mg of IV metoclopramide.² That study did not bode well for magnesium. Some lower-quality studies conducted since then were a little more encouraging. One study found 1 g of IV magnesium was superior to placebo but only in the subset of patients who had migraines with aura (ie, resolving neurologic symptoms).³ Another study pitted 1 g IV magnesium against 8 mg IV dexamethasone or 10 mg of IV metoclopramide and found that magnesium reduced pain better than the other agents. While this all would seem to add up to modest support for magnesium in some patients, the studies were small and some had issues with methodology. That’s why the American Headache Society makes no recommendation on the use of magnesium for emergency management of headaches, but states that it may be of benefit in patients with migraine with aura.

Lastly, we reviewed evidence on the use of IV magnesium in the management of asthma in the emergency department. The most important insight to remember when evaluating a study of asthma (or any study, in all fairness) is to focus primarily on patient-centered outcomes. Measuring peak flow rates in patients with asthma may give us some sense of the clinical picture, but there are major problems with this tool. First, it is effort related. The outcome and accuracy of the measurement rely on the patient’s willingness and skill to perform the test. Sometimes peak flow measurements are falsely low because patients still feels poorly, and so they simply do not blow with enough force. What we care about are patient-centered outcomes, such as whether a patient required an admission to the hospital. In this regard, the use of IV magnesium early in the emergency department visit appears to have a consistent benefit in decreasing the number of patients who will end up needing admission to the inpatient units. This is true in both adults and children.^4,5 Magnesium seems to benefit those with more severe symptoms. The number of patients needed to treat to decrease an admission seems to range from two to six. Even using the conservative estimate of six is impressive. While the quality of the study designs was variable, the signal seems consistent.

And we can’t forget the uses of magnesium that are noncontroversial. So we gave a shout out to magnesium for polymorphic ventricular tachycardia, seizures from eclampsia, and electrolyte replacement as well. If there are more uses for this favorite FOAMed element, we’ll be sure to cover them in the future.

Dr. Faust (@JeremyFaust) is a clinical instructor at Harvard Medical School and an attending physician in department of emergency medicine at Brigham & Women’s Hospital, Boston, Massachusetts.

Dr. Westfafer (@Lwestafer) is an attending physician and research fellow at Baystate Medical Center, clinical instructor at the University of Massachusetts Medical School in Worcester, and co-host of FOAMcast.