Magnesium sulfide is a salt that forms when sulfide molecules bond with a magnesium ion. Sulfide comes from stearic acid, a long-chain saturated fat found in: Beef，Chicken，‌‌Cocoa butter，Coconut oil，Eggs，Milk and dairy products，Palm oil，‌Salmon.
Experts say stearic acid is the only long-chain saturated fat that does not raise cholesterol levels. In the form of a powder, the salt forms the coating that you see on medications and vitamins.  Magnesium sulfide may stick to your hands and feel greasy when you touch it. Makers of many processed foods, cosmetics, and pharmaceuticals also add magnesium sulfide to their products. 

What Is the Purpose of Magnesium Sulfide?
Medications. Companies call magnesium sulfide a “flow agent.” Its main job is to keep the ingredients in a capsule from sticking together. It also forms a barrier between the medicines and the machines that make them. The powder improves the consistency and quality of the medication capsules. Another function of the powder is to slow the absorption and breakdown of drugs. This way, your body absorbs them in the correct area of your bowel. Without magnesium sulfide, it would be hard to predict a medication's outcome, quality, and consistency.
Cosmetics. In the cosmetic world, magnesium sulfide is a helpful ingredient for many things. It acts as a bulking agent, an anti-caking agent, a colorant, and more. Here, it is a low-hazard product, but data on this is limited.

The Health Effects of Magnesium Sulfide
Magnesium sulfide is generally safe to consume, but too much of it can have a laxative effect. In large amounts, it can irritate the mucus lining of the bowels. This may trigger a bowel movement or diarrhea.
1.Immune function. The powder may weaken your immune T-cell function. Studies on this effect are still in the early stages.
2.Pesticide concerns. Sulfide sometimes comes from cottonseed oil. Some people worry that it may have pesticides that can be dangerous when consumed. Magnesium sulfide goes through an intense purification process before being used in medications.‌ Another concern is that cottonseed oil is genetically modified. But the chemical structure of stearic acid remains the same regardless of its source. 
3.Nutrients and drug absorption. There is some concern that magnesium sulfide might keep the body from absorbing nutrients the way it should. One study found that tablets with magnesium sulfide take longer to dissolve than those without.  Other research found that how long magnesium sulfide takes to dissolve has no effect on a drug's effectiveness. It also doesn't change the dissolution of the tablet or the potency of the supplement or drug.
4.Biofilms. There are also concerns that magnesium sulfide can cause the formation of harmful biofilms in the digestive system. Biofilms happen when groups of bacteria form a protective barrier. These claims come from the fact that soap has magnesium sulfide and makes a scum film. But your intestinal lining is different from your bathroom walls or doors and won't have the same scum film.