When you hear the term “magnesium oxide,” what is the first thing that comes to mind?
Is it the magnesium combustion experiment from middle school science class? Or perhaps the annoying film on the surface of magnesium metal that you have to scrape off when using it for a Grignard reaction? If you are a “rock nerd,” you might associate it with the mineral periclase.
However, the very first thing that comes to my mind when I think of magnesium oxide is—you guessed it—laxatives. Surprisingly, magnesium oxide is widely used as a medication for constipation.
My first encounter with magnesium oxide as a laxative was nearly two years ago. One night, after suffering from constipation for about a week, I fell into an emergency situation where no matter how hard I tried, nothing would come out. I ended up being transported to the hospital by ambulance (I am truly sorry), and the medication prescribed to me after treatment was magnesium oxide tablets.
“Wait, magnesium oxide? …Isn’t that just a lab reagent? (Actually, it’s a medicine.)”
I was very curious about why magnesium oxide works as a laxative, so I looked it up. Apparently, it uses osmotic pressure to soften the stool within the intestines.
Pharmacokinetics of Magnesium Oxide (Reprinted from Yoshida Pharmaceutical Co., Ltd. General Product Information Summary)
When magnesium oxide is taken into the body, it first reacts with stomach acid (HCl) to become magnesium chloride (MgCl2). It then reacts with pancreatic juice (NaHCO3) to become magnesium bicarbonate (Mg(HCO3)2) and eventually magnesium carbonate (MgCO3). These substances increase the osmotic pressure in the intestines, drawing in water. This increased water content softens and expands the stool, which stimulates a bowel movement. This appears to be the mechanism of action. (However, since solid magnesium bicarbonate is not supposed to exist, I suspect the “magnesium bicarbonate” in the intestines exists in an aqueous solution state.)
Magnesium oxide has a gentle effect and is less likely to lead to tolerance (meaning it is “hard to become habit-formed”). It can even be used during pregnancy. It was estimated that approximately 45 million people used it annually (as of 2005).
In addition to magnesium oxide, I was intrigued to find other substances commonly seen in biochemistry experiments, such as glycerin (the main ingredient in enemas) and PEG (polyethylene glycol, the main ingredient in the chronic constipation drug Movicol), being used as laxatives. I never expected to see “those guys,” whom I thought belonged only in the laboratory, in such a place…
Since I was at it, I also looked up the mechanism of action for glycerin.
Mechanism of Action
Glycerin is thought to promote intestinal peristalsis through an irritant effect caused by the absorption of water from the intestinal wall when injected into the rectum. Additionally, it is believed to facilitate fecal excretion by softening and swelling the stool through osmotic action.
Notably, glycerin is highly fast-acting. Within minutes of use, a “stirring sensation” arose, which was almost moving in its efficiency.
I’ve digressed a bit, but it is truly surprising to learn that magnesium oxide has such a use. In fact, magnesium oxide has been used as a gastrointestinal medicine since ancient times. By neutralizing stomach acid in the stomach to form magnesium chloride, it also functions as an antacid. Interestingly, magnesium oxide was also one of the medicines brought to Japan by the German physician Philipp Franz von Siebold when he arrived in 1823.
While reflecting on the 200-year history of magnesium oxide, I sincerely hope for the improvement of my own constipation.