The myth
Melasma and post-inflammatory hyperpigmentation are basically the same thing. They both cause dark spots. The treatment is the same. If a product works for one, it will work for the other.
Why people believe it
Because they can look almost identical, especially on the face. Both present as brown or dark brown patches. Both affect the cheeks, forehead, and upper lip. Both get worse with sun exposure. If you are looking at two dark marks and trying to figure out what they are based purely on colour and location, these two can be very hard to tell apart.
Most people do not get a formal diagnosis before they start treating. They see dark spots, they search for solutions, and they land on a routine built around exfoliation, brightening actives, and maybe a stronger treatment. That routine is usually designed around PIH, because PIH is the type most people have heard of and the one most product marketing is aimed at.
If you happen to have PIH, that approach can work. If you actually have melasma, the same approach often makes things worse.
How they are actually different
PIH is a memory. It is the mark left behind after a specific inflammatory event: a breakout, a burn, a cut, an irritating product, an eczema flare. The event happened, the melanocytes in that area responded by producing excess pigment, and the visible mark is the record of it. The trigger is over. The mark is fading on its own timeline. Treatments that speed turnover and suppress melanin production work with the body's natural clearing process.
The key thing about PIH is that it is usually a surface-layer problem with a surface-layer solution. The trigger was a one-time event. The pigment is a deposit being cleared. A good topical routine can do most or all of the work.
Melasma is an active signalling problem. It is not tied to a single event. The melanocytes in affected areas are in a state of chronic overactivity driven by signals coming from multiple directions: hormones, inflammation, UV, visible light, heat, vascular activity. Many of those signals originate below the epidermis, in the hormonal and inflammatory environment that surrounds the melanocytes from the inside.
This is the fundamental difference, and it is the one that matters most for treatment. PIH is a mark being cleared. Melasma is a system being driven. Surface treatments can clear marks. They cannot calm a system that is being activated from inside.
What goes wrong when you treat melasma like PIH
The standard PIH approach is: exfoliate to accelerate turnover, use brightening actives to suppress melanin, and wait for the skin to cycle through. It assumes the pigment is a fixed deposit being gradually cleared.
With melasma, that assumption does not hold. The pigment is not a fixed deposit. It is being actively produced. And the aggressive exfoliation that helps PIH can trigger the inflammatory response that makes melasma worse.
Higher-concentration acids on melasma-affected skin can produce a temporary brightening effect (the surface looks better after the top layer sheds) followed by a flare (the inflammation from the acid triggers the already-hyperactive melanocytes to produce more pigment). The person sees initial improvement, pushes harder, and watches the melasma darken. The conclusion is usually that the products are not strong enough. The reality is that the approach is wrong for the type.
Melasma treatment is slower, more conservative, and more focused on calming the signals than on removing the pigment. Because those signals are partly internal, managing melasma often means addressing what is happening inside the system, not just what is being applied on top of it. The hormonal environment, systemic inflammation, oxidative stress, the metabolic inputs that keep melanocytes in overdrive: these sit below the epidermis, in the layer that topical treatments can only partially reach.
This is why melasma has a reputation for being "untreatable." It is not untreatable. It is mistreated, because the standard approach only addresses one layer of a two-layer problem.
How to tell which one you have
The single most useful question is: can you trace it to a specific event?
If the dark mark appeared after something identifiable (a breakout, an injury, a procedure, a rash) and it sits in the exact footprint of that event, PIH is the most likely answer.
If the pigmentation appeared without a clear triggering event, sits in a symmetrical pattern across both sides of the face, fluctuates with hormones or seasons, and does not consistently improve with treatment, melasma is more likely.
Some people have both. A melasma-prone person who gets a breakout can develop PIH on top of an existing melasma pattern. That makes identification harder, but the underlying principle still applies: the melasma component will not respond to the PIH approach, and treating it that way risks worsening the part that needs the most careful handling.
If you are not sure, a dermatologist can usually distinguish them based on distribution pattern, Wood's lamp examination, and trigger history. Getting the identification right before choosing a strategy saves months of misdirected effort.
Melasma and PIH look the same on the surface. Underneath, PIH is a mark being cleared. Melasma is a system being driven. The treatment has to match the mechanism, and for melasma, that mechanism runs deeper than the surface.
