Hydroquinone
In plain terms
A skin-lightening agent used to fade dark spots and melasma. It works, but it is now a prescription drug in the US and banned from cosmetics in the EU, because long-term unsupervised use can cause a hard-to-reverse skin darkening called ochronosis.
What the evidence shows
Hydroquinone lightens skin by blocking the enzyme that makes pigment, and dermatologists still consider it one of the most effective treatments for melasma and stubborn dark spots. The problem is what happens with prolonged or unsupervised use. The signature adverse effect is exogenous ochronosis, a blue-black darkening of the treated skin that is the opposite of what the user wanted and is often hard to treat. A systematic review of reported cases found the risk climbs with strength and time, mostly above 4% and past three months of continuous use. Under careful dermatology supervision the risk is low: one review of more than 10,000 supervised US exposures over decades counted only 22 cases. The cancer question gets overstated. High oral doses produced some tumors in rodents, but IARC places hydroquinone in Group 3, not classifiable as a human carcinogen, and the systemic dose from topical use is no greater than what comes from coffee and some fruit. The real reasons it left the shelves are ochronosis and years of people using it without supervision. The FDA pulled it from over-the-counter sale in 2020, leaving one prescription combination product; the EU banned it from skin-lightening cosmetics back in 2001. Gentler options for the same job, including azelaic acid, kojic acid, arbutin, tranexamic acid, niacinamide, and vitamin C, carry none of this baggage.
On the label
Listed as hydroquinone. If you find it in an over-the-counter cream, fade serum, or an imported skin-lightening product, that product is being sold outside US drug rules, and some imports carry it at high strength without naming it clearly. Use it only under a dermatologist, at low strength, for short stretches, and stop if any darkening appears.