PRESERVATIVE
Methylchloroisothiazolinone
- preservative
A very potent rinse-off preservative used at trace levels. It is a known contact allergen for a small number of people.
- products in our library
- 208
- of researched products
- 5%
- on the label, in our shampoos
- usually #21
- most common in
- Shampoo
- in water
- Water-soluble
At a glance
Four named levels, read from this ingredient’s chemical class, its charge, its solubility, or where it sits on the labels we have. More dots means more of the property named — never “better”.
How much of it is left on hair between washes.
What it takes to get it off the hair again.
What it is in the formula to protect.
Where it sits on the labels we have, in the category it appears in most.
| Property | Level | Out of |
|---|---|---|
| Buildup risk | Low | 1 of 5 |
| Rinse-out behaviour | Rinses clean | 5 of 5 |
| Role | Keeps water-based formulas | 4 of 5 |
| Prominence | Secondary | 2 of 5 |
What it does for a formula
What Methylchloroisothiazolinone does
The general function first, then how the role changes by product type where we have researched it.
A potent rinse-off preservative used at very low levels; a contact allergen for some.
Water alone removes it, so it does not accumulate between washes.
Charge
Charge is not recorded for this ingredient.
Solubility
Rinses out with water alone — it does not accumulate on hair between washes.
Where it appears
Where it appears
One row per product category in our library: how many products list it, what share of that category that is, and where it typically sits on the label.
| Product type | How common | Typical position on label ? |
|---|---|---|
Shampoo
93 products
|
secondary · #21 | |
Conditioner
73 products
|
secondary · #16 | |
Co-wash
18 products
|
secondary · #23 | |
Mask / treatment
17 products
|
secondary · #22 | |
Leave-in
4 products
|
secondary · #15 | |
Styler: curl cream
2 products
|
secondary · #20 | |
Scalp treatment
1 product
|
secondary · #26 |
| Product type | Products | Share of category | Median position | Earliest position | Band |
|---|---|---|---|---|---|
| Shampoo | 93 | 11% | 21 | 11 | secondary |
| Conditioner | 73 | 10% | 16 | 9 | secondary |
| Co-wash | 18 | 18% | 23 | 10 | secondary |
| Mask / treatment | 17 | 4% | 22 | 11 | secondary |
| Leave-in | 4 | 1% | 15 | 8 | secondary |
| Styler: curl cream | 2 | 2% | 20 | 12 | secondary |
| Scalp treatment | 1 | 1% | 26 | 26 | secondary |
No product-type differences have been researched yet — the reading below is computed from where it sits on the labels in our library.
Position footprint
Every listing in our library, by position on the label.
| Positions | Zone | Listings |
|---|---|---|
| 1-3 | Leading | 0 |
| 4-8 | Supporting | 1 |
| 9-15 | Secondary | 59 |
| 16-25 | Lower | 105 |
| 26+ | Trace | 75 |
How to read this
US labelling rules put ingredients above 1% in descending order of amount; below 1% they may be listed in any order.
So position tells you about relative amount only above that line, and we never convert a position into a percentage.
Why it matters to shoppers
What this ingredient is evidence of, what it is not evidence of, and what our own method can and cannot tell you.
- a preservative that works at very low levels
- a product that keeps over its shelf life
- a rinse-clean feel
- an exact safety score — we do not publish one, for any ingredient
- how a finished product performs by itself — that is the whole formula, not one line of it
- the exact percentage in a bottle — label position never converts into a number
- a secondary isothiazolinone, listed below the main ingredients in our shampoos
- it was on the label of every product we counted — we read printed ingredient lists, not marketing copy
- that the product would be better without one
- that a product is right or wrong for you
We do not publish safety scores or hazard grades. Where a regulator has published a restriction or a use condition, we state it and cite it; otherwise we describe what the ingredient does and stop.
Why some people avoid it
A preservative that is effective at very low concentrations, which is why it appears near the end of an ingredient list.
Methylisothiazolinone and its chloro- form caused a documented epidemic of allergic contact dermatitis across Europe after the permitted concentration was raised in the 2000s. This is the best-evidenced concern on any of these pages: the effect was measured in patch-test clinics, and the regulation changed in response.
Where regulators stand: The EU banned methylisothiazolinone from leave-on cosmetics in 2017 and cut the rinse-off limit. It remains permitted in rinse-off products in both the EU and the US at restricted concentrations.
A regulator has acted on this
Browse products in our library without Methylchloroisothiazolinone
Commonly paired with
Ingredients that appear alongside this one most often in our library. Near-universal ingredients are excluded — they pair with everything and tell you nothing.
- Methylisothiazolinone · 207
- Sodium Chloride · 104
- Cetyl Alcohol · 91
- Benzyl Alcohol · 86
- Dimethicone · 74
- Cocamidopropyl Betaine · 72
- Disodium EDTA · 68
- Glycol Distearate · 65
- Stearyl Alcohol · 63
- Tetrasodium EDTA · 62
The number beside a name is how many products in our library list both.
These ingredients tend to appear together to keep the formula stable, soften the strand, and carry the formula and let it dry.
Found in these products
The most prominent placements first. The badge is the position on that product’s own ingredient list.
Taliah Waajid · Leave-in
Soft & Curly For Natural Hair
#8 on the label · $1.00/oz
Pantene · Conditioner
Pro-V Classic Clean Conditioner
#9 on the label · $0.40/oz
Creme of Nature · Conditioner
Chamomile and Comfrey Nourishing Conditioner
#10 on the label · $0.47/oz
Paul Mitchell · Conditioner
Tea Tree Special Conditioner
#10 on the label · $3.34/oz
Rusk · Conditioner
Puremix Activated Charcoal Purifying Conditioner
#10 on the label · $1.58/oz
Simply Shea · Co-wash
Deep Conditioner & Co-wash with Organic Shea Butter
#10 on the label · $0.97/oz
Taliah Waajid · Leave-in
for Children Tangles Out Today Leave-in Conditioner
#10 on the label · $1.00/oz
TRESemme · Conditioner
TRESemme Botanique Nourish Conditioner
#10 on the label · $0.23/oz
Equate · Shampoo
Equate Beauty Moisture Rich Shampoo with Vitamin E
#11 on the label · $0.10/oz
Head & Shoulders · Mask / treatment
Head and Shoulders Royal Oils Deep Moisture Masque Conditioner Treatment
#11 on the label · $1.45/oz
Head & Shoulders · Conditioner
Head and Shoulders Supreme Color Protect Conditioner with Argan Oil and Manuka Honey
#11 on the label · $1.39/oz
Head & Shoulders · Conditioner
Head and Shoulders Supreme Nourish and Smooth Conditioner with Argan Oil and Jojoba Oil
#11 on the label · $1.17/oz
See all 208 products in the library that list Methylchloroisothiazolinone
Identity
What this entry is, and what our references call it.
Methylchloroisothiazolinone
- CAS 26172-55-4
- PubChem 33344
- isothiazolinone
- methylchloroisothiazolinone
For your hair?
Whether Methylchloroisothiazolinone matters for your hair depends on your hair.
the 8th most-listed preservative or antioxidant in our libraryWhat this ingredient does, and where it sits on the labels we have researched.
A verdict on any product that contains it, and never a safety grade.
We do not physically test your hair. Everything above is read from the ingredient lists in our library and from published references — the consultation is what turns it into a plan.
References
Sourced Backed by a manufacturer or regulatory source.
This ingredient
- Manufacturer or regulatory PubChem — Compound identity: CID 33344, CAS 26172-55-4, C4H4ClNOS · accessed 2026-08-28
How this page was made
- Every number above is counted from the ingredient lists in our own product library — 208 of 3889 researched products list this ingredient. Nothing on this page is written per ingredient.
- The dots are named levels read from an ingredient’s chemical class, charge, solubility, or where it sits on the labels we have — never a score.
- US labelling rules put ingredients above 1% in descending order of amount; below 1% they may be listed in any order. Position tells you about relative amount only above that line, and we never convert a position into a percentage.
- The prominence bands (leading, supporting, secondary) are the same ones the product pages use to read a single label, applied here to the median position across a category.
- We do not publish safety scores or hazard grades. Where a regulator has published a restriction or a use condition, we state it and cite it; otherwise we describe what the ingredient does and stop.
- Identifiers and references appear only when a published source is attached to this entry; where none is, the row is simply absent rather than filled in.
- This is cosmetic information, not medical advice, and nothing here diagnoses or treats a health condition.
- Entry last updated 2026-09-02. Product-page methodology: the product library.