Methylchloroisothiazolinone, also known as MCI or Methylisothiazolinone (MI), is a synthetic chemical compound commonly used as a preservative in skincare products including creams, lotions, shampoos, conditioners and cleansers. Its primary use is to reduce the growth of bacteria, fungi and other microorganisms, which can extend the shelf life of products and prevent spoilage. MCI/MI has faced recent restrictions because of its potential to cause skin irritation and allergic reactions. Most skincare manufacturers now choose alternative preservatives or lower concentrations in their formulas.