Magnesium lotion, often called magnesium oil, is a topical preparation that delivers magnesium to the body using a concentrated solution of magnesium chloride or magnesium sulfate applied directly to the skin. This method bypasses the digestive tract, offering an alternative to oral supplements. The central question is whether this application can still trigger the systemic side effect most commonly associated with oral magnesium: diarrhea.
The Laxative Mechanism of Oral Magnesium
High doses of oral magnesium cause loose stools due to the osmotic effect. When magnesium is ingested, a portion is absorbed through the small intestine, but a significant amount, particularly in forms like citrate, oxide, or sulfate, remains unabsorbed. These unabsorbed ions create an osmotic gradient, pulling water from surrounding tissues and blood vessels into the intestinal lumen.
This influx of water increases the volume and fluidity of the contents in the colon, softening the stool and promoting a bowel movement. Magnesium-based laxatives are intentionally formulated to maximize this effect, often using poorly absorbed salts like magnesium sulfate. The intensity of the laxative effect depends on the dose and the specific chemical form of the magnesium salt consumed.
Transdermal Absorption of Magnesium
Transdermal absorption involves magnesium ions moving through the skin layers into the underlying tissues and bloodstream. The stratum corneum, the skin’s outermost layer, acts as the primary barrier, regulating what substances can enter the body. Magnesium, a charged mineral ion, does not easily diffuse through this lipid-rich layer.
Absorption occurs through hair follicles and sweat glands, which offer microscopic channels that bypass the stratum corneum barrier. Factors that enhance delivery include the lotion’s concentration, the duration of contact, and the surface area of application. Transdermal delivery avoids the first-pass metabolism of oral supplements, delivering the mineral directly into the systemic circulation. This route utilizes magnesium chloride, which is highly soluble and readily separates into ions for absorption.
Absorption Levels and GI Impact
Systemic absorption through topical magnesium application is slow and limited compared to a high-dose oral supplement. While transdermal delivery raises cellular magnesium levels, the rate at which the mineral enters the bloodstream is much lower than the rapid influx caused by a laxative dose. Since absorbed magnesium bypasses the intestinal tract, the mechanism causing diarrhea—the presence of unabsorbed ions in the gut—is circumvented.
For diarrhea to occur, a sufficiently high concentration of magnesium must remain in the intestine to generate the necessary osmotic pull. The small amount of magnesium entering the system via the skin does not cycle back into the gut to trigger this laxative effect. Therefore, the magnesium reaching the bloodstream from topical application is insufficient to cause the digestive distress associated with oral use. Gastrointestinal side effects remain primarily linked to the oral route.
Common Localized Reactions to Topical Magnesium
While magnesium lotion is unlikely to cause diarrhea, users frequently report localized reactions at the application site. The most common experiences include temporary itching, tingling, or a mild burning sensation. This is often due to the high salt concentration of the magnesium chloride solution, which can feel irritating on sensitive skin or as it dries.
Application to broken skin intensifies this burning feeling because exposed nerve endings react to the salt. The tingling may be more pronounced in individuals with low cellular magnesium levels, but this tends to decrease with consistent use. To mitigate these reactions, users can dilute the lotion with water or a standard body moisturizer, or apply it to less sensitive areas.