Methylchloroisothiazolinone and methylisothiazolinone (MCI/MI) is a broad-spectrum antimicrobial preservative blend used at very low concentrations to prevent bacterial and fungal growth in rinse-off and (formerly) leave-on cosmetic products. Due to its high sensitizing potential, its use has become restricted in many regions.
| Reported effect | How often | Notes |
|---|---|---|
| Allergic contact dermatitis | Common | A well-recognized and increasingly prevalent contact allergen, particularly with leave-on products. |
| Skin irritation, redness, and itching | Common | Especially on sensitive or compromised skin or at higher concentrations. |
| Eczematous flares and chronic dermatitis | Uncommon | May occur in sensitized individuals with repeated exposure. |
| Facial or eyelid dermatitis | Uncommon | Associated with cosmetics, wipes, and personal care products. |
| Airborne contact dermatitis | Rare | Reported from exposure to paints or products containing the preservative. |
| Severe or widespread allergic reactions | Rare | Documented in highly sensitized patients. |
Frequencies reflect typical cosmetic use reported in the literature, not a guarantee for your skin.
Peer-reviewed papers on this ingredient, via PubMed.