Ingredient dictionary

Methylchloroisothiazolinone

Also listed as: MCI

A preservative from the isothiazolinone family that keeps water-based products free from bacteria and mold during storage and use. It is on the label of 11 products in our library. Checked against our library November 6, 2026.

The catch: mCI belongs to the MI/MCI family, and MI is a leading cause of preservative contact allergy, so anyone with a known MI allergy should avoid this ingredient, and the EU has banned MI in leave-on products.

MI / MCI. MI is a leading cause of preservative contact allergy, and the EU has banned it in leave-on products. Anyone with a known MI allergy needs to avoid it.

What Methylchloroisothiazolinone does

Stops bacteria and mold from growing
Extends shelf life of water-based formulas
Protects formulas from spoiling during use

Methylchloroisothiazolinone, usually shortened to MCI, is a synthetic preservative from the isothiazolinone group. It is made in a chemical plant, not taken from plants or minerals, and it is usually sold as a blend with MI, a close relative.

Formulators use it because it works in small amounts to stop bacteria and mold from growing in water-based products. Its main home is rinse-off products like shampoo and conditioner, where it helps keep the formula stable between uses. In the finished product it adds little scent, color or texture of its own.

Good for

Suits rinse-off hair and skin products, for people with no known isothiazolinone allergy.

Be careful if

MCI belongs to the MI/MCI family, and MI is a leading cause of preservative contact allergy, so anyone with a known MI allergy should avoid this ingredient, and the EU has banned MI in leave-on products.

Frequently asked questions

Is methylchloroisothiazolinone safe to use?

For most people, it is well tolerated in rinse-off products like shampoo. It can trigger allergic reactions in some people, though, especially anyone already sensitive to MI. If you have a known isothiazolinone allergy, skip it and check labels.

What are the signs of an MI or MCI allergy?

A contact allergy, a skin reaction from touch, usually shows as itching, redness, or a rash where the product touched the skin, sometimes after repeated use. If you notice these signs, stop using the product and talk with a doctor or dermatologist.