DMDM Hydantoin: The Preservative That Makes Its Own Formaldehyde
Starting in 2020, women began filing suit against Unilever over TRESemme haircare, claiming DMDM hydantoin in the formulas caused hair loss and scalp injuries. The filings argued the company had known since at least 2012 that this class of preservative could cause scalp irritation and shedding.
Unilever didn’t concede the science, and it didn’t lose. The consolidated case was dismissed with prejudice in December 2022, after an earlier ruling had already stripped out most of the claims because the plaintiffs hadn’t alleged that they personally were allergic to the preservative or had reacted to it. There is no TRESemmé settlement, whatever the ads on law-firm websites suggest. The $10.2 million figure that circulates alongside this story belongs to a different Unilever case entirely, over a Suave keratin product recalled in 2012.
The reformulation is the part worth noticing anyway. Companies change formulas for all sorts of reasons and a quiet ingredient swap admits nothing, but the ingredient is still perfectly legal and still sitting in plenty of other bottles.
Short answer: DMDM hydantoin preserves shampoo by slowly releasing formaldehyde, and formaldehyde is a known human carcinogen. That classification comes from breathing it at industrial levels for years, which is not what a bottle of shampoo does to you. The real, well-evidenced problem is allergy: if you’re already sensitized to formaldehyde, this ingredient will find you. It’s legal and uncapped in the US, and legal in the EU capped at 0.6% with a warning label.
What it’s actually doing in there
Shampoo is mostly water, and water grows things. Every bottle needs a preservative or it turns into a science experiment in your shower. So this is a real job, not a gratuitous additive.
DMDM hydantoin does that job in an unusual way. It doesn’t kill microbes directly. It slowly breaks down in the bottle and releases small amounts of formaldehyde, and the formaldehyde does the preserving.
That’s the entire issue in one sentence. Formaldehyde is classified by the International Agency for Research on Cancer as a known human carcinogen, and this ingredient exists to generate a steady supply of it.
The word “formaldehyde” appears nowhere on the label. It doesn’t have to, because it isn’t an ingredient. It’s a product the ingredient makes after you’ve bought it.
How much, and does it matter
This is where I want to be careful, because it’s easy to make this sound worse or better than it is.
The amounts released are small, and the strongest evidence for formaldehyde causing cancer in humans comes from inhaling it at industrial levels over years, not from rinse-off products. Anyone telling you a shampoo will give you cancer is overstating what’s known.
What’s better established at consumer levels is irritation and allergy. Formaldehyde is a well-recognized contact allergen, and formaldehyde-releasing preservatives are a common cause of scalp and face dermatitis. If you’re already sensitized to formaldehyde, these products can set you off. That’s the mechanism the lawsuits leaned on.
The EU takes the exposure seriously enough to regulate it twice over. DMDM hydantoin is capped at 0.6% in cosmetics, and any product releasing formaldehyde above a threshold has to say so on the label. That threshold was tightened sharply in 2022, from 0.05% down to 0.001%, because Europe’s own scientific committee concluded the old one didn’t protect people who were already sensitized. The US has no equivalent requirement at all.
The names, because there are a lot of them
This is the practical part, and it’s harder than it should be, because “formaldehyde releaser” is a category rather than a single ingredient.
DMDM hydantoin is the most common in shampoo. Quaternium-15 is the one most likely to cause an allergic reaction, and it isn’t close: in North American patch-test data it lands in the top handful of allergens tested, several times DMDM hydantoin’s rate. It is not, despite how often you’ll read it, the biggest formaldehyde releaser. When somebody actually measured all of them side by side, diazolidinyl urea released more than either. Then there’s imidazolidinyl urea, sodium hydroxymethylglycinate, and 2-bromo-2-nitropropane-1,3-diol, which usually appears as bronopol.
Quaternium-15 is banned outright in the EU. DMDM hydantoin isn’t, and you’ll see it claimed that it is. That’s wrong: it sits on Europe’s approved preservative list with a 0.6% cap.
None of them look like anything. That’s the problem. No shared prefix. No family resemblance. Unless you already know the list, they read as more unpronounceable filler in a long ingredient panel.
If you only memorize one, make it DMDM hydantoin, because it’s the one you’re most likely to meet in haircare.
| Quick facts | DMDM hydantoin |
|---|---|
| Also written | 1,3-dimethylol-5,5-dimethylhydantoin, or the trade name Glydant |
| What it does | Preservative, working by slowly releasing formaldehyde |
| Typical use | Suppliers say 0.1% to 0.6%; industry reports 0.074% or less in practice |
| EU status | Legal, capped at 0.6%, warning label required above 0.001% released |
| US status | Legal, no cap, no warning required |
| EWG | No numeric score published any more. Allergy risk rated high |
| Who it matters for | Anyone already patch-test positive to formaldehyde |
Who should care most
Anyone with an itchy, flaking or sore scalp that hasn’t been explained. This class of preservative is among the more common causes, and it’s a cheap thing to rule out by switching products for a month.
Anyone already patch-test positive for formaldehyde. That one isn’t a maybe. It’s a known trigger.
Anyone using these products daily on a small child, where the same amount lands on a much smaller body.
And it’s worth knowing that the exposure isn’t evenly spread. A 2025 study from the Silent Spring Institute had 70 Black women and Latinas in Los Angeles log every personal care product they used. Over half were using at least one product with a formaldehyde releaser in it, and DMDM hydantoin was the most common one by a distance, turning up in 58% of the hair products flagged. If you’ve wondered why this ingredient comes up so often in conversations about hair relaxers and scalp health, that’s the reason.
And if none of that is you, this still lands somewhere on a sensible avoid list, just further down it than the internet suggests. A preservative you rinse off in ninety seconds is a smaller exposure than the fragrance on the same label, which stays on your hair all day.
What to buy instead
This one’s easy to avoid. Essentially every clean haircare brand already skips it, so you don’t need to hunt.
Innersense is a B Corp and the pick if you want salon performance from a short ingredient list. Rahua is the luxury end, with sourcing you can actually trace. Acure carries The Climate Label and is often under $12, which makes this a free switch rather than an expensive one. Attitude is EWG Verified, and EWG Verification specifically excludes formaldehyde releasers, so the mark does the checking for you.
That last point is the shortcut worth remembering. Rather than memorizing six chemical names, look for a certification whose standard already bans the category. We’ve sorted the brands that clear it in the best clean shampoo and conditioner.
Frequently asked questions
Is DMDM hydantoin safe?
It’s legal and widely used, and the amounts of formaldehyde released are small. The clearer risk at consumer levels is skin sensitization: formaldehyde is a recognized contact allergen and these preservatives are a common cause of scalp and facial dermatitis. The cancer classification comes from long-term high-level inhalation, which isn’t what rinse-off shampoo represents.
Does DMDM hydantoin cause hair loss?
That was the claim in lawsuits filed against Unilever over TRESemme products starting in 2020, and TRESemme in the US dropped the ingredient by July 2022. The company didn’t concede the science, and a reformulation isn’t proof. What is well established is that these preservatives can cause scalp irritation and dermatitis, which is not the same thing as hair loss but can accompany it.
Which ingredients are formaldehyde releasers?
DMDM hydantoin, quaternium-15, imidazolidinyl urea, diazolidinyl urea, sodium hydroxymethylglycinate, and bronopol (2-bromo-2-nitropropane-1,3-diol). They share no naming pattern, which is what makes them hard to spot. Quaternium-15 releases the most.
Is it banned in Europe?
Not banned, but regulated more tightly than in the US. The EU caps DMDM hydantoin at 0.6% in cosmetics and requires products releasing formaldehyde above a low threshold to say so on the label. There’s no equivalent US labeling rule, so an American bottle won’t warn you.
Related reading
The rest of this series: are sulfates bad for your hair, cocamidopropyl betaine, and silicones in hair products. For brands that skip this entirely, see the best clean shampoo and conditioner. Our guide to the skincare ingredients worth avoiding covers the rest of the label, and every brand named has a scored profile in the brand directory.
Sources: International Agency for Research on Cancer, Formaldehyde, IARC Monographs Volume 100F. European Commission, Cosmetic Products Regulation 1223/2009, Annex V. Bloomberg Law, Unilever lawsuit over chemicals in TRESemme products dismissed (Castillo v. Unilever, N.D. Ill., dismissed with prejudice December 2022). Lv C, Hou J, Xie W, Cheng H, Investigation on formaldehyde release from preservatives in cosmetics, International Journal of Cosmetic Science 2015;37(5):474-478. de Groot AC, White IR, Flyvholm MA, Lensen G, Coenraads PJ, Formaldehyde-releasers in cosmetics, part 2, Contact Dermatitis 2010;62(1):18-31. Dodson RE, Franklin ET, Zota AR, et al., Formaldehyde and formaldehyde releasing preservatives in personal care products used by Black women and Latinas, Silent Spring Institute, 2025.
