If you have the urge to order Mag for a patient, follow your intuition. But Mag might be even more important when replacing potassium.

When treating hypoK+, if the patient has an IV or will have one, I just order Mag 2g IV rapid infusion (never need to do it slowly, 20 minutes is great) along with 60meq oral K. If they are below 2.0 K+, I either give 2 K runs or if they need/can tolerate volume, a liter of D51/2NS with 20meqK (K runs which are painful and seem to take forever to get to the bedside). Of note, people with CHF very often Mag depleted from diuretics and other etiology.

If you give the Mag IV and K po a the same time, Mag is hitting them first. Tough ones are when you don’t have an IV, because Mag oxide is trash. Sometimes I still order it. But people with no IV are likely not very sick and probably have K above 3.

Something might happen when K gets below 3. So if below 3, I usually give some IV, some po.

Dr Harmon asked me about this before she graduated, and I went looking for a few papers on it. I did a little lit search and cant find a true RCT of K repletion VS K repletion WITH or AFTER Mag repletion. It would be expensive to do and lots of confounders and no one wants to spend $ on a Mag study because it’s an old, cheap medication. **(Although some press coverage now on a Magnesium L-Threonate [which I take] study that showed cognitive benefit in Alzheimers patients).

In the few relevant papers I found, authors just generally recommend Mag with K. But we should always be careful when doing something that is logical but isn’t proven empirically. Sometimes things that make sense in theory don’t pan out in studies (vitamin C in sepsis).

One study in ICU patients compared those getting lots of Mag vs those getting none, and looked at their K balance. They found an obvious benefit to Mag repletion for K balance. They cite lots of basic science research on the K-Mg interplay.

At the end of the day, most people are Mg deficient and people with low K maybe even more likely Mg deficient. Mag is awesome, no downside, patient might flush or get sleepy. Just watch out in those with renal failure.