Ingredient review
Methylisothiazolinone
INCI: Methylisothiazolinone
A powerful but controversial preservative that is a common allergen; many prefer to avoid it.
In plain English
Methylisothiazolinone (MI) is a synthetic preservative that stops bacteria, yeast, and mold from growing in your shampoo, conditioner, and other water-based products. It is very effective at low levels, but it is also one of the most common causes of allergic contact dermatitis—a red, itchy rash. Because of this, it has been heavily restricted in Europe and is increasingly avoided by many brands.
Review score
Safety, usefulness, and evidence
Higher-caution ingredient
The evidence base is relatively strong for its common cosmetic role.
Main practical flags: irritation is high; clogging risk is low.
- Source
- synthetic
- Evidence
- strong
- Irritation
- high
- Clogging risk
- low
Quick decision guide
Read the cautions before using
Methylisothiazolinone can be useful, but watch for high irritation potential.
Plain-English read
Treat this as a practical screening step before you compare products that contain this ingredient.
- Step 1Start with the score, then check the irritation and clogging risk before judging Methylisothiazolinone.
- Step 2Use the "Best for" and "Use caution if" sections to match the ingredient to your skin, not just to a marketing claim.
- Step 3If a product stings, breaks you out, or worsens irritation, judge the finished formula and stop using it even if the ingredient scores well.
Score terms in plain English
Irritation risk
highMore likely to cause dryness, stinging, peeling, or reactivity if used too aggressively.
Clogging risk
lowLess likely to feel heavy or contribute to clogged pores for most skin types.
Evidence level
strongThere is a stronger practical or research basis for the ingredient role described here.
What it is
Methylisothiazolinone is a small, synthetic molecule that acts as a broad-spectrum preservative. It belongs to the isothiazolinone family, which also includes methylchloroisothiazolinone (MCI). It is often used alone or in combination with MCI.
How it works
It works by disrupting the cell membranes and metabolic processes of microorganisms, effectively killing them or stopping them from multiplying. This keeps the product safe from contamination during use.
Pros
Powerful preservation
Works at very low levels to protect products from microbial contamination, extending shelf life.
Cost-effective
Inexpensive for manufacturers, which can keep product prices lower.
Cons and cautions
Common allergen
One of the most frequent causes of allergic contact dermatitis, especially in leave-on products like wipes and moisturizers.
High irritation potential
Even for non-allergic people, it can cause stinging, redness, or irritation on sensitive skin.
Best for
- People with no history of contact allergies to preservatives
- Those using rinse-off products with very low concentrations
Use caution if
- Anyone with sensitive skin, eczema, or a known allergy to isothiazolinones
- People using leave-on products like moisturizers or wipes
Usage tips
Safety summary
Methylisothiazolinone is a potent preservative with a well-documented risk of causing allergic contact dermatitis. It is considered safe for most people at very low concentrations in rinse-off products, but those with sensitive skin or a history of allergies should avoid it entirely, especially in leave-on formulas.
Research notes
Numerous clinical studies and dermatology reviews confirm MI as a leading cause of contact allergy. Patch testing data from Europe and North America show sensitization rates of 2-6% in the general population. Regulatory agencies have responded with restrictions.
Common label clues
- Typical concentration
- Typically 0.0015% to 0.01% in leave-on products; up to 0.0015% in rinse-off products in the EU
- Regulatory status
- In the EU, MI is banned in leave-on products and restricted to 0.0015% in rinse-off products. In the US, the FDA allows it in cosmetics but the industry is moving toward voluntary restrictions. Always check local regulations.
- Common uses
- Shampoos, Conditioners, Body washes, Wipes, Sunscreens, Moisturizers
- Environmental note
- Isothiazolinones can be toxic to aquatic life at low concentrations. They are not readily biodegradable and may persist in water systems.
Good to know
- In the EU, MI is banned in leave-on cosmetics and limited to 0.0015% in rinse-off products.
- It is often paired with methylchloroisothiazolinone (MCI) in a 3:1 ratio.
- The American Contact Dermatitis Society named MI the 'Allergen of the Year' in 2013.
Common questions
What is Methylisothiazolinone in beauty products?
Methylisothiazolinone (MI) is a synthetic preservative that stops bacteria, yeast, and mold from growing in your shampoo, conditioner, and other water-based products. It is very effective at low levels, but it is also one of the most common causes of allergic contact dermatitis—a red, itchy rash. Because of this, it has been heavily restricted in Europe and is increasingly avoided by many brands.
What does Methylisothiazolinone do in a beauty product?
It works by disrupting the cell membranes and metabolic processes of microorganisms, effectively killing them or stopping them from multiplying. This keeps the product safe from contamination during use.
Is Methylisothiazolinone safe for most people?
Methylisothiazolinone is a potent preservative with a well-documented risk of causing allergic contact dermatitis. It is considered safe for most people at very low concentrations in rinse-off products, but those with sensitive skin or a history of allergies should avoid it entirely, especially in leave-on formulas.
Who should be careful with Methylisothiazolinone?
Anyone with sensitive skin, eczema, or a known allergy to isothiazolinones People using leave-on products like moisturizers or wipes
Research sources
Ingredient reviews are educational and are not medical advice. Patch test new products and ask a licensed clinician about persistent irritation, allergies, pregnancy-specific questions, or diagnosed skin conditions.