PRESERVATIVE
Methylisothiazolinone
- preservative
A powerful rinse-off preservative used at very low levels; a known contact allergen for some people.
- products in our library
- 389
- of researched products
- 6%
- on the label, in our shampoos
- usually #22
- most common in
- Shampoo
- in water
- Water-soluble
At a glance
Four things worth knowing about how this ingredient behaves on hair.
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 Methylisothiazolinone 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, so it does not build up 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
197 products
|
secondary · #22 | |
Conditioner
121 products
|
secondary · #18 | |
Mask / treatment
30 products
|
secondary · #22 | |
Co-wash
20 products
|
secondary · #23 | |
Leave-in
7 products
|
secondary · #21 | |
Styler: gel
6 products
|
secondary · #13 | |
Scalp treatment
2 products
|
secondary · #25 | |
Styler: curl cream
2 products
|
secondary · #21 | |
Finisher: anti frizz finish
1 product
|
secondary · #28 | |
Relaxer
1 product
|
secondary · #27 |
2 more product types
| Product type | How common | Typical position on label ? |
|---|---|---|
Styler: mousse
1 product
|
secondary · #12 | |
Styler: volumizer
1 product
|
secondary · #22 |
| Product type | Products | Share of category | Median position | Earliest position | Band |
|---|---|---|---|---|---|
| Shampoo | 197 | 13% | 22 | 11 | secondary |
| Conditioner | 121 | 11% | 18 | 10 | secondary |
| Mask / treatment | 30 | 5% | 22 | 8 | secondary |
| Co-wash | 20 | 20% | 23 | 11 | secondary |
| Leave-in | 7 | 1% | 21 | 9 | secondary |
| Styler: gel | 6 | 2% | 13 | 7 | secondary |
| Scalp treatment | 2 | 1% | 25 | 23 | secondary |
| Styler: curl cream | 2 | 1% | 21 | 13 | secondary |
| Finisher: anti frizz finish | 1 | 3% | 28 | 28 | secondary |
| Relaxer | 1 | 25% | 27 | 27 | secondary |
| Styler: mousse | 1 | 0% | 12 | 12 | secondary |
| Styler: volumizer | 1 | 1% | 22 | 22 | secondary |
We have not researched differences by product type yet. The reading below comes 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 | 2 |
| 9-15 | Secondary | 60 |
| 16-25 | Lower | 206 |
| 26+ | Trace | 121 |
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 seeing this ingredient tells you, what it does not, and the limits of our own method.
- 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, which depends on the whole formula, not one line of it
- the exact percentage in a bottle, because label position never converts into a number
- a secondary isothiazolinone, listed below the main ingredients in our shampoos
- it was on the printed ingredient list of every product we counted (we read labels, 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 Methylisothiazolinone
Commonly paired with
The ingredients that most often appear alongside this one in our library. We leave out the near-universal ones, because they pair with everything and tell you nothing.
- Methylchloroisothiazolinone · 370
- Sodium Chloride · 202
- Cocamidopropyl Betaine · 156
- Dimethicone · 142
- Disodium EDTA · 135
- Glycol Distearate · 134
- Guar Hydroxypropyltrimonium Chloride · 132
- Tetrasodium EDTA · 129
- Cetyl Alcohol · 126
- Benzyl Alcohol · 122
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, clean and build foam, and smooth the surface.
Found in these products
The most prominent placements first. The badge is the position on that product’s own ingredient list.
Bumble and bumble · Styler
Gel
#7 on the label · $7.40/oz
Kenra · Mask / treatment
Nourishing Masque
#8 on the label · $5.29/oz
Suavecito · Styler
Firme Hold Styling Gel
#9 on the label · $1.37/oz
Taliah Waajid · Leave-in
Soft & Curly For Natural Hair
#9 on the label · $1.00/oz
Pantene · Conditioner
Pro-V Classic Clean Conditioner
#10 on the label · $0.40/oz
Suave · Conditioner
Tropical Coconut Conditioner
#10 on the label · $0.09/oz
BioSilk · Styler
Silk Therapy Glazing Gel
#11 on the label · $2.86/oz
Creme of Nature · Conditioner
Chamomile and Comfrey Nourishing Conditioner
#11 on the label · $0.47/oz
Equate · Shampoo
Kids 3-in-1 Body, Hair & Bubble Bath, Lavender
#11 on the label · $0.14/oz
Paul Mitchell · Conditioner
Tea Tree Special Conditioner
#11 on the label · $3.34/oz
Rusk · Conditioner
Puremix Activated Charcoal Purifying Conditioner
#11 on the label · $1.58/oz
Simply Shea · Co-wash
Deep Conditioner & Co-wash with Organic Shea Butter
#11 on the label · $0.97/oz
See all 389 products in the library that list Methylisothiazolinone
Identity
What this entry is, and what our references call it.
No public-domain structure depiction is available for this entry.
Methylisothiazolinone
- isothiazolinone
- methylisothiazolinone
For your hair?
Whether Methylisothiazolinone matters for your hair depends on your hair.
the 6th 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 comes from the ingredient lists in our library and from published references. The consultation is what turns it into a plan.
References
Seed entry, sourcing in progress Written from our reference work; primary sourcing is still being attached.
No published source is attached to this entry yet.
How this page was made
- Every number above is counted from the ingredient lists in our own product library: 389 of 6816 researched products list this ingredient. Nothing on this page is written per ingredient.
- The dots are levels, not scores. Each one comes from what kind of ingredient it is (its chemical class, charge and solubility) or from where it sits on the labels we have.
- 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. Without one, we leave the row out rather than fill it in.
- This is cosmetic information, not medical advice, and nothing here diagnoses or treats a health condition.
- Entry last updated 2026-09-22. Product-page methodology: the product library.
