Magnesium repletion is very common in hospitalized patients. Is there any difference between PO and IV magnesium in patients without symptoms of hypomagnesemia?
Dosage Forms
There are a lot of magnesium-based dosage forms approved in the United States. Two of the most common are magnesium sulfate (IV) and magnesium oxide (PO):
| Salt Form | Route | % Magnesium | Typical Dose |
| Magnesium sulfate | IV | 20% | 2 gm = 200 mg elemental Mg |
| Magnesium oxide | PO | 60% | 400 mg = 240 mg elemental Mg |
As you can see, a “typical” dose of magnesium sulfate and magnesium oxide contain similar amounts of elemental magnesium. Of course, because magnesium oxide is taken orally, bioavailability must be considered. One must also consider a potential for diarrhea with oral magnesium, particularly given that oral magnesium is actually used for constipation in some OTC products (eg, magnesium citrate laxative).
Clinical Data
There is a retrospective chart review comparing oral and intravenous magnesium therapy, although the article has some significant limitations for clinical applicability.1
This study compared varying doses of oral magnesium oxide (800-1600 mg, N=164) to a single dose of IV magnesium sulfate (2 gm, N=188). As would be anticipated, the baseline characteristics were very different between the two groups. The oral magnesium group received about three times the dose of elemental magnesium (1130 mg vs. 400 mg), had higher baseline magnesium levels (1.78 vs. 1.64 mg/dL), and had repeat serum levels drawn much earlier following magnesium replacement (8.25 vs. 18 hrs).
The primary endpoint of the study (change in serum magnesium level after 6 to 24 hours) was greater with IV therapy than any dose of oral therapy (mean change 0.24 mg/dL vs. 0.05-0.11 mg/dL, p=0.003). Linear regression showed that the following factors were significantly associated with a greater change in magnesium level:
- IV route of administration
- Shorter time between magnesium dose and rechecking serum level
- Renal dysfunction
- Concomitant use of loop diuretics
Putting It Into Perspective
Despite the limitations of the previously mentioned study, it does demonstrate some interesting points about magnesium replacement:
- Oral magnesium replacement is probably best given as chronic therapy. In the study, all oral magnesium doses increased serum magnesium similarly (a median of 0.1 mg/dL). This may indicate a saturable absorption process, where a multi-dose oral regimen may be better than the traditional single-dose repletion strategy.
- You are a magnesium replacement wimp. In the study, 2 grams of IV magnesium sulfate increased serum levels by a paltry median of 0.2 mg/dL. If you’re serious about repleting magnesium, don’t be afraid to reach for big doses — consider 3 or 4 grams.
- Rechecking too soon may give a false sense of security. Because magnesium is primarily an intracellular cation, there may be a delayed distribution phase from the plasma into the cells. It is unclear whether rechecking early results in higher magnesium levels because of a less time for intracellular distribution or less time for urinary excretion.
References
- Reed BN, Zhang S, Marron JS, Montague D. Comparison of intravenous and oral magnesium replacement in hospitalized patients with cardiovascular disease. Am J Health Syst Pharm. 2012;69(14):1212-7. PMID 22761075.
Photo by intropin
Since renal reabsorption of magnesium is saturable, it would be ideal to administer IV magnesium over longer infusion times. In the cited study the authors do not mention the infusion time but if it is like our institution it would be over 1-2 hours, which is probably not optimal.
Interesting thought — it would be interesting to see a comparative study between long vs. short infusion times. To my knowledge, this was the only magnesium replacement article I could find at all — it’s clearly an understudied area of medicine where personal experience and comfort dominates practice patterns.
The magnesium vial you show at the top is Magnesium sulfate 50%. (0.5 gm/ml) Wouldn’t that supply 500 mg instead of the 200 mg you have in your chart?
The table has been updated to reflect a typical dose (2 gram of magnesium sulfate) contains 16.25 mEq of elemental magnesium, which is equivalent to about 200 mg of elemental magnesium. Usually magnesium sulfate is given as a diluted rider — the image of the vial isn’t necessarily reflective of a typical magnesium dose or dosage form.
This reference is wrong. 1 gm (1000 mg) of Magnesium Sulfate injection contains 100 mg of elemental Magnesium. So a 1gm/2ml MGSO4 injection would contain 50 mg of elemental Magnesium per ml, not 100 mg/ml.
You’re correct — the table has been updated to reflect a typical dose (2 gram of magnesium sulfate) contains 16.25 mEq of elemental magnesium, which is equivalent to about 200 mg of elemental magnesium.