What to Know About Methylchloroisothiazolinone Allergies

Medically Reviewed by Stephanie S. Gardner, MD on May 21, 2023
3 min read

Methylchloroisothiazolinone is a preservative in many skin care products that stops fungi, yeast, and bacteria from growing. Many people are allergic to it or can become so. It must be used in low concentrations.

Methylchloroisothiazolinone is a chemical used in skin care products, household cleaners, and industrial products as a preservative. It is usually mixed with methylisothiazolinone and the combination is sometimes called Kathon CG or Euxyl K 100.

It was first used in Europe in the 1970s and the United States in the 1980s. The original European recommendation was to use 0.003% concentration or 30 parts per million (ppm). This led to many allergic reactions. The recommendation then changed to 15 ppm in wash-off products and 7.5 ppm in leave-on cosmetics.

In 2000, companies started using methylisothiazolinone in industrial products. By 2005 some cosmetic companies were using as much as 50 to 100 ppm. There were reports of more reactions to methylchloroisothiazolinone, methylisothiazolinone, and the combination of the two.

Some countries like Canada don’t allow methylchloroisothiazolinone as a single ingredient. It must be combined with methylisothiazolinone and can only be used in rinse-off products at a concentration of 15 ppm.

Today you will often find methylchloroisothiazolinone in skin care products and cosmetics, including:

  • Shampoo
  • Hair dye
  • Bleach
  • Sunscreen
  • Baby wipes
  • Eyeliner
  • Blush
  • Face powder
  • Makeup remover
  • Nail polish
  • Waxing products
  • Soaps
  • Baby soap and shampoo

You’ll also see it in many cleaning and industrial products, such as:

  • Paint
  • Glue
  • Laundry soap
  • Dishwashing soap
  • Stain remover
  • Fabric softener
  • Glass cleaner
  • Washing powder

Methylchloroisothiazolinone can cause skin problems when it’s used in large amounts in different products. This is called sensitization. It engages your immune system and irritates your skin. Once this happens you may become permanently allergic to methylchloroisothiazolinone the next time you are exposed to it.

Watch for irritations and allergic reactions to methylchloroisothiazolinone on your:

  • Face
  • Hands
  • Scalp
  • Legs
  • Feet
  • Neck

Reactions in these areas usually come from certain products. Reactions to methylchloroisothiazolinone are mostly caused by:

  • Sunscreen
  • Shampoo
  • Conditioner
  • Wet work with cleaning products

These skin and allergic reactions usually look like contact dermatitis. Your symptoms could include:

  • Redness
  • Swelling
  • Raised bumps or hives
  • Red patches on lighter skin
  • Grey, purple, or brown patches on darker skin
  • Itchiness
  • Flaky skin
  • Eye swelling
  • Peeling skin

Some products use terms like “hypoallergenic” or “for sensitive skin.” This doesn’t mean you won’t have a reaction. There aren’t any legal rules for using these terms.

The best way to avoid a reaction if you are allergic is to read labels and avoid products that use methylchloroisothiazolinone. You might also find it listed under other names like:

  • Kathon CG
  • Kathon 886
  • Kathon WT
  • Kathon LX
  • MC/MCI-886
  • 5-chloro-2-methyl-4-isothiazolin-3-one
  • 5-chloro-20methyl-3[H2]-isothiazolinone

If you get a rash or allergic reaction after using products with methylchloroisothiazolinone, stop using that product right away. Treat the reaction as soon as possible. Here are a few ways to do so.

Emollient. The first step is to wash your skin fully and then use an emollient. These are ointments or lotions that soften and moisturize your skin. Apply a thick layer over the area that’s irritated right after you have a shower or you get your hands wet. Don’t rub it in as this can irritate your skin even more.

Antihistamines. You can buy antihistamine medicine over-the-counter at your pharmacy. These will help ease itchiness and your rash.

Steroid cream.Corticosteroid creams can help relieve allergic reactions and skin rashes. Sometimes you can buy a low-dose over-the-counter cream at your pharmacy. If you need something stronger, your doctor will need to prescribe it for you.

To use a steroid cream:

  • Apply an emollient to your skin first.
  • Wait 30 minutes and then apply a thin layer only where you have a rash or irritation.
  • Apply up to twice per day and only in small amounts.
  • Stop using it as soon as the rash goes away. See your doctor if it doesn’t improve.

Steroid creams can also have side effects if you use too much cream and for too long. These can include:

  • Acne
  • Extra hair growth
  • Thin skin
  • Changes in your skin color

Steroid pills. You might need to take steroid pills if you have a severe reaction to methylchloroisothiazolinone or if it covers a lot of your skin.

Anyone can get a rash if they use a product with too much methylchloroisothiazolinone. Talk to your doctor if you have skin problems or suspect an allergy. They can do tests to confirm it and recommend other products to use.