Ingredient dictionary

Methylisothiazolinone

Also listed as: MI

Methylisothiazolinone, often shortened to MI, is a synthetic preservative that stops bacteria, mold and yeast from growing in water-based products. It is on the label of 18 products in our library. Checked against our library November 6, 2026.

The catch: MI is a leading cause of preservative contact allergy, the EU has banned it in leave-on products, and anyone with a known MI allergy needs to avoid it.

MI / MCI. Methylisothiazolinone and methylchloroisothiazolinone, isothiazolinone preservatives.

What Methylisothiazolinone does

Keeps water-based formulas fresh longer
Stops bacteria and mold from growing
Protects products against spoilage during use

Methylisothiazolinone is made in a lab, not taken from plants or minerals. It belongs to a group of preservatives called isothiazolinones, which are built around a small ring of atoms. Formulators use it because water-based products, such as shampoo, can grow bacteria and mold during everyday use.

In a formula, it is used in small amounts and is typically mixed into the water phase. It has no scent of its own and does not add oil or weight, so it is rarely noticed by the user. It is often paired with other preservatives, which is common practice in cosmetic formulas.

Good for

Formulas people rinse off, like shampoo, cleanser and conditioner, where it helps keep water-based products stable.

Frequently asked questions

Is methylisothiazolinone banned?

The EU has banned it in leave-on products such as creams and leave-in conditioners. Rinse-off products like shampoo are treated differently, and rules vary by region.

How do I know if I'm allergic to methylisothiazolinone?

Signs of contact allergy can include redness, itching or a rash where the product touched the skin. If you suspect a reaction, stop using the product and speak with a doctor or dermatologist, who can arrange patch testing.