Dear internet people: please don't do this to yourselves. I sometimes wonder if internet browsing makes people's skin better, since they have easy access to research, or worse, because they also have easy access to people's crackpot theories about what they should put on their faces.
Y'all already know that there are a fair number of home remedies that make me cringe. Milk of Magnesia is the perfect example of this. Although there are some elements that may be helpful, it's only helpful if you ignore the damage you may be inducing.
Milk of Magnesia is magnesium hydroxide suspended in water with a bit of sodium hypochlorite. Magnesium hydroxide is an inorganic compound patented in 1818 for digestive issues. It is formed by a simple salt metathesis reaction in which magnesium salt is added to ammonium hydroxide.
Mg2+ + 2OH- ---> Mg(OH)2
It is most commonly used as an antacid or as a laxative. When used as an antacid, the OH- groups will pop on off the molecule and bind to the extra H+s you have in your painful, acidic belly, leaving you with water. This prevents your stomach's hydrochloric acid from reaching your gastrointestinal nerves, meaning that you won't be in horrible pain. Hooray! Science! When used as a laxative, you rely on Milk of Magnesia's osmotic force. The magnesia ions formed after the OH- groups pop off aren't absorbed by your intestinal tract, leaving you with a high concentration of magnesium ions. Osmosis, at its most basic, is a movement of water from areas with low concentrations of ions to areas with high concentrations of ions. Thus, the magnesium pulls fluid into the intestines. Your colon sexily responds by dumping its contents, triggering you to dump yours. Hooray! Bowel movements!
The second key ingredient of Milk of Magnesia, sodium hypochlorite (NaClO), is standard household bleach, which is an oxidizing agent. It's in a very low concentration (doctors don't usually recommend drinking large quantities of bleach), but it is a very effective buffering agent.
Why the fuck do people want to put laxatives on their faces?
Against all odds, there actually is a pretty good reason why people put Milk of Magnesia on their faces. A study by Stewart and Downing (1981) showed that magnesium hydroxide is remarkably effective at breaking down wax esters and steryl esters extracted from human skin. These are the major components of the oil on your face.
In other words, it really actually is a de-greaser. It's no surprise that people keep recommending this as an option for a primer. It definitely would minimize oiliness and I have no doubt that it has the potential to prolong your makeup wear.
Why is this a terrible, very bad, no-good idea?
Remember how magnesium hydroxide has a bunch of OH- groups ready to jump out at you and neutralize any acid that may come its way? That means that Milk of Magnesia is alkaline as fuck. It has a pH of 10.5. For comparison, ammonia has a pH of 11. Baking soda looks downright harmless in comparison, with a pH of only 8.3.
In order to ward of particularly unpleasant bacteria, your skin needs to maintain a mildly acidic pH. P. acnes, for example, is best inhibited by a pH between 4.2 and 5.6. Your skin has evolved to maintain this balance very effectively. Your sebaceous glands secrete a thin layer of acidic film called the acid mantle, which protects you from viruses, bacteria, and other potential threats. Milk of Magnesia was formulated to neutralize stomach acid. It's going to do that, but on your face. This totally destroys your acid mantle, leaving your skin unprotected. Using Milk of Magnesia as a primer sounds like the perfect way to turn your face into a nasty-bacteria's dream.
Interfering with the acid mantle may also cause contact dermatitis, interfere with the activation of enzymes involved in extracellular lipid processing, impede your skin's ability to shed its dead layers, and damage overall skin integrity.
When you use Milk of Magnesia as an everyday primer, you are coating your whole face in this shit on a daily basis. This has the potential to be quite damaging. Milk of Magnesia may be effective for oil control, but there are a shitload of primers that can help control oil that are actually supposed to go on your skin. I would recommend buying one of those, instead.