Resorcinol
In plain terms
A coupler in permanent hair dye and an ingredient in some acne and scalp treatments. Its signature concern is the thyroid: it is a suspected hormone disruptor. European regulators judge it safe at the low, restricted levels used in hair dye, but it is worth limiting, especially in pregnancy and for frequent users.
What the evidence shows
Resorcinol is used in permanent hair dyes as a coupler, the partner that reacts with the main dye and peroxide to set color, and it turns up in some acne and dandruff treatments. Its standout concern is thyroid hormones: in the lab it blocks the enzyme that builds thyroid hormone, and in animals at high doses it is goitrogenic, meaning it can enlarge the thyroid. It sits on the European list of suspected endocrine disruptors, and France pushed to classify it as a substance of very high concern over thyroid and fetal-development worries, though the EU shelved that move when the living-organism evidence came back mixed and partly reversible. Set against that, Europe's safety committee assessed resorcinol at the low, restricted levels allowed in hair dye and found a wide safety margin, around 400 times the required cushion, so an occasional adult user is not the worry. Skin allergy is well known in theory but uncommon in practice. So this is a depends: occasional permanent color at the restricted levels is regulator-cleared, while the thyroid suspicion, its exclusion from EWG-verified products, and caution in pregnancy or with frequent salon-level exposure all point to limiting it. The strong thyroid and nervous-system effects in the literature come from high oral or occupational doses, not the trace in a dye. If you want to steer clear, resorcinol-free hair color exists.
On the label
Listed as resorcinol or 1,3-benzenediol. Fine for most adults at the levels used in hair dye, but if you are pregnant, color often, or work with dyes, it is a reasonable one to limit, and resorcinol-free dyes are available.