Hydroquinone works best alone, on a schedule, with sunscreen. Most problems come from helping it too much.
Hydroquinone slows the enzyme that makes pigment. It is prescription strength in most places, it is typically used in cycles rather than indefinitely, and it is unforgiving of two things: sun exposure, which undoes the progress, and irritation, which can itself trigger more pigment in deeper skin tones.
Arbutin, kojic acid, tranexamic acid, and azelaic acid all target pigment too. On top of prescription hydroquinone they mostly add irritation risk without being necessary. Dermatologists do sometimes combine them, deliberately and at chosen strengths. Doing it yourself is where the trouble starts. Ask before adding any of them.
Hydroquinone plus a prescription retinoid (often with a mild steroid) is a well-established melasma treatment. If that is what you were prescribed, follow the schedule exactly. If you were prescribed hydroquinone alone, do not add a retinoid on your own; ask.
If a dark-spot serum and a prescription cream both contain hydroquinone, you are doubling a prescription-strength dose. Use only what your dermatologist prescribed.
Generally yes, and many dermatologists like the pairing (vitamin C in the morning under sunscreen, hydroquinone at night). If your skin is sensitive, introduce one at a time.
If you also use benzoyl peroxide, the two can react and cause a temporary discoloration. Separate them and mention it to your dermatologist. Any persistent darkening is a reason to stop and call.
That is a prescriber decision. It is usually used in cycles with breaks, partly to reduce the risk of a rare paradoxical darkening with very long continuous use.
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