Quaternium-15
In plain terms
A preservative that works by slowly releasing formaldehyde. It is the most common cause of preservative allergy in cosmetics, and the EU has banned it outright. The cancer worry from the released formaldehyde is small at cosmetic levels, but the allergy problem is real, so it is better to avoid.
What the evidence shows
Quaternium-15 is a formaldehyde-releasing preservative: it keeps products from spoiling by steadily giving off small amounts of formaldehyde, which does the antimicrobial work. That mechanism is the heart of the concern. Quaternium-15 is the most common cause of preservative-related allergic contact dermatitis in cosmetics, testing positive in a few percent of patch-tested people and as high as around 7 percent in North America, and it sensitizes both through the formaldehyde it releases and on its own. The EU prohibited it in cosmetics in 2019, though it remains legal in North America, and some large brands dropped it years earlier under public pressure. On the cancer question, which people often raise: formaldehyde is a known carcinogen when inhaled at high doses, but cancer experts note that the amount released from cosmetics is low and that avoiding quaternium-15 has not been shown to change cancer risk, so the practical reason to skip it is allergy, not cancer. Because it is a leading, avoidable contact allergen and plenty of gentler preservatives exist, this is one to avoid, especially if you already react to formaldehyde or other releasers.
On the label
Listed as quaternium-15 or Dowicil. Common in older or budget lotions, shampoos, and cleansers. Worth avoiding, and important to avoid if you are allergic to formaldehyde, since reacting to one formaldehyde releaser often means reacting to the others. Products sold in the EU will not contain it.