Methylisothiazolinone

Methylisothiazolinone and pregnancy

Safe to Use

• Common allergen
• Irritant

Methylisothiazolinone is a preservative used to prevent microbial contamination in cosmetic products. While it has no known reproductive toxicity, it is a potent contact allergen that can cause skin irritation and allergic reactions. From a pregnancy perspective, MI does not pose risks to fetal development. It has minimal systemic absorption and no evidence of teratogenic effects. However, pregnancy can increase skin sensitivity, making allergic reactions more likely or more severe. Many dermatologists recommend avoiding known irritants and allergens during pregnancy to prevent discomfort and skin barrier disruption. While not explicitly contraindicated in pregnancy, the EU ban on MI in leave-on products and the AAD's general recommendation to avoid it suggests pregnant women may want to choose products without this preservative, especially if they have sensitive skin or a history of contact dermatitis.

Methylisothiazolinone and breastfeeding

Safe to Use

• Common allergen
• Irritant

Methylisothiazolinone is a preservative with minimal systemic absorption that remains primarily on the skin's surface. Its main concern is contact allergy rather than internal exposure. While MI does not pose risks to milk production or breastfed infants, breastfeeding mothers should be cautious about using products containing this ingredient on or near the breast area. Contact dermatitis on the breast could lead to discomfort during breastfeeding and potentially allow the ingredient to come into contact with the infant's skin. Additionally, postpartum hormonal changes can increase skin sensitivity. The AAD and dermatologists recommend avoiding MI when possible due to high allergy rates. Breastfeeding mothers, particularly those with sensitive skin or eczema, may want to choose preservative systems less likely to cause irritation.

Methylisothiazolinone and postpartum

Safe to Use

• Common allergen
• Irritant

Methylisothiazolinone is a preservative known for causing contact allergic reactions. While it has no specific postpartum concerns, postpartum hormonal changes can increase skin sensitivity and reactivity. Women with sensitive skin or a history of allergies may want to avoid MI during the postpartum period when skin can be more reactive. The preservative's high allergen potential makes it less ideal for this sensitive time.

Methylisothiazolinone and trying to conceive

Safe to Use

Methylisothiazolinone is a preservative with no known effects on fertility or reproductive hormones. It has minimal systemic absorption and no evidence of reproductive toxicity. Women trying to conceive can use products containing MI without concern for fertility effects. However, those with sensitive skin may still want to avoid it due to allergy potential.

Methylisothiazolinone and PCOS

Safe to Use

• Common allergen
• Irritant

Methylisothiazolinone is a preservative with no hormonal activity or endocrine-disrupting properties. It does not affect androgen levels, insulin sensitivity, or other PCOS-related metabolic factors. The primary concern with MI for PCOS is its allergenic and irritant potential. Women with PCOS who experience acne or skin inflammation may find that irritating ingredients like MI worsen their skin condition. However, this is an individual sensitivity issue rather than a PCOS-specific concern. While MI does not directly impact PCOS, those with sensitive or reactive skin may want to avoid it. The European ban on MI in leave-on products suggests choosing alternative preservatives when possible.

Methylisothiazolinone and general use

Caution Advised

• Common allergen
• Irritant

Methylisothiazolinone is a preservative that gained popularity as an alternative to parabens but has since become highly controversial due to exceptionally high rates of allergic contact dermatitis. It's effective at preventing microbial growth in cosmetics but can cause severe skin reactions in sensitized individuals. The main concern with MI is its status as one of the most common contact allergens in cosmetics. The American Contact Dermatitis Society named it Contact Allergen of the Year in 2013. Studies show sensitization rates have increased dramatically, with reactions ranging from mild irritation to severe chemical burns. Once sensitized, individuals may react to extremely low concentrations. The SCCS found the ingredient unsafe in leave-on products due to unacceptable sensitization risks. The EU banned MI in leave-on cosmetic products in 2017 and restricts it to 0.01% in rinse-off products. The FDA has not banned it, but the AAD and many dermatologists strongly recommend avoiding products containing MI, especially for those with eczema, sensitive skin, or a history of contact allergies. Many brands have voluntarily removed it from formulations.

Everything else flagged for the same profiles

Related ingredients

Ingredients suggested in place of Methylisothiazolinone, each one with its own page here.

• Phenoxyethanol
• Sodium Benzoate
• Potassium Sorbate

Ingredients that raise the same concerns.

• Parfum/FragranceIrritant, Common allergen
• Lavandula Angustifolia OilIrritant, Common allergen
• Melaleuca Alternifolia Leaf OilIrritant, Common allergen
• EugenolIrritant, Common allergen
• MethylchloroisothiazolinoneIrritant, Common allergen

Where this came from

Truva summarises published guidance from bodies including ACOG, the FDA, the EU SCCS and the Cosmetic Ingredient Review. It has no access to your medical history and it is not a substitute for your doctor, dermatologist or midwife. These assessments have not been reviewed by a clinician, and nothing here is a diagnosis or a prescription. If you are pregnant, breastfeeding or being treated for PCOS, take the ingredient list to the person looking after you.

Assessment last pulled from the ingredient database on 15 September 2026.