Diazolidinyl Urea
In plain terms
A preservative that works by slowly releasing formaldehyde. It gives off the most formaldehyde of its group and is a frequent cause of skin allergy. It is not the same as plain urea. Formaldehyde-free options exist.
What the evidence shows
Diazolidinyl urea is a preservative that works by slowly releasing small amounts of formaldehyde, which keeps bacteria and fungi out of a product over time. That mechanism is also the problem. Of the formaldehyde-releasing preservatives, diazolidinyl urea gives off the most formaldehyde and has the strongest tendency to sensitize skin, meaning it can set off allergic contact dermatitis, and once someone reacts to it they often react to formaldehyde and to the related releasers too. In European patch-test data, somewhere between half a percent and one and a half percent of tested people show an allergy to it. The main concern at the levels used on skin is this contact allergy rather than the cancer risk tied to breathing formaldehyde, which is a different exposure route; the safety panel allows diazolidinyl urea up to a low cap and considers it usable there, mostly limited by the irritation it can cause. Still, it is an avoidable ingredient. It shares part of its name with plain urea but is a separate, synthetic molecule, and gentler preservatives that release no formaldehyde cover the same job. For anyone with sensitive or allergy-prone skin in particular, it is a straightforward one to skip.
On the label
Listed as diazolidinyl urea. It is a formaldehyde-releasing preservative and the heaviest releaser of the group, and a common cause of preservative allergy. It is not the same as plain urea. Formaldehyde-free preservatives do the same job.