Methylisothiazolinone is a strong preservative that stops bacteria and fungi growing in water-based products. It is now mostly found in rinse-off products such as shampoos and body washes because it is a well-known cause of skin allergy.
- Also known as
- MI, MIT, Methylchloroisothiazolinone (MCI), a closely related preservative often used tog, MCI/MI, Methylchloroisothiazolinone and methylisothiazolinone, Isothiazolinones
- Pregnancy
- Generally considered fine in pregnancyIt is used at very low levels as a preservative and there is no specific known pregnancy concern, but ask your doctor if you are unsure or have had a reaction to it before.
- Evidence
- Well established
What it does
Products that contain water can grow bacteria, yeast and mould once opened, especially in a hot, humid climate. Methylisothiazolinone kills these microbes at very small amounts, which keeps the product safe to use. It is often paired with a related preservative, methylchloroisothiazolinone, to cover a wider range of microbes. It does nothing for your skin itself. Its only job is to protect the product. The downside is that it is one of the more common causes of allergic skin reactions among preservatives. Because of this, some countries no longer allow it in leave-on products such as creams and wet wipes, and limit how much can be used in rinse-off products.
Good for
- Keeping water-based products free from microbes
- Rinse-off products such as shampoos, conditioners and body washes
- Most skin types when used briefly and rinsed off, unless you are allergic to it
How to use it
You do not need to do anything special. It is already in the product at a very low level to keep it fresh. It is mostly used in products you wash off, where contact with your skin is short. If you know you are allergic to it, check ingredient lists carefully, including on wet wipes, hair products, and household products such as cleaning sprays and some paints. Rules on how much is allowed differ between countries, so imported products may not all follow the same limits.
Who should be careful
Methylisothiazolinone can cause allergic contact dermatitis. This is an itchy, red, sometimes swollen or blistering rash. It often shows on the face, eyelids, neck or hands, and it can appear a day or more after use, so the cause is easy to miss. Once you become allergic, the allergy usually lasts for life, and even small amounts can set it off. Leave-on products and wet wipes carry a higher risk than rinse-off products. If you get a rash that keeps coming back, stop using the product and see a doctor or dermatologist, who can do a patch test to find the cause. On darker skin, an allergic rash can leave dark patches that take weeks or months to fade, so it is worth dealing with a reaction early. People with eczema or very reactive skin may want to choose products without it.
