The good news is that this is almost never about hygiene, and it usually isn't permanent. Most cases have surprisingly simple explanations once you know what you're looking at, and a fair few turn out not to be pigmentation in the first place.

Here's what actually causes dark underarm skin, how to work out which one applies to you, and what genuinely helps.

Dark underarms aren't always pigmentation

Before you go looking for a brightening product, it's worth knowing that three quite different things get grouped together under "dark armpits".

Pigment. Actual melanin in the skin, produced in response to irritation, friction or hormones. This is what most treatments are aimed at.

Shadow. Hair sitting just beneath the surface, visible through the skin even when you've shaved clean. Nothing to do with pigment at all.

Texture. Skin that has thickened and taken on a velvety, slightly raised quality, which reads as darker even where the colour hasn't changed much.

They can look similar in the mirror, but they respond to completely different things. Working out which one you're dealing with is the most important first step, because a cream aimed at melanin won't touch a shadow, and no amount of exfoliation will help with thickening that has a medical cause.

Common causes of dark armpit skin

Friction

Skin that gets rubbed repeatedly responds by producing more melanin. Tight sleeves, weight changes, a bag strap sitting in the same spot every day, arms that meet the body when you walk. It's the same mechanism that darkens inner thighs and the back of the neck, and it builds slowly enough that most people never connect it to anything.

Shaving

Razors are a double problem. The blade drags across the skin, which is friction, and the process leaves micro-cuts that trigger a small inflammatory response each time. Repeated often enough over years, that inflammation can leave pigment behind.

Shaving also creates the shadow effect, which is a separate thing altogether and comes up below.

Post-inflammatory hyperpigmentation

This is the big one, and it's the mechanism sitting behind most of the others.

When skin is irritated, it produces extra melanin as part of healing. Once the irritation stops, that pigment stays behind and fades slowly. Anything that repeatedly annoys the skin under your arms can cause it: waxing, ingrown hairs, heat rash, an allergic reaction, or a product that stings a little every morning.

It's more common and more persistent in deeper skin tones, because more active melanocytes mean a stronger pigment response to the same irritation. If that's your skin, the priority isn't finding something to lighten the area. It's working out what keeps setting the inflammation off, because pigment can't fade while the trigger is still there.

Deodorant irritation

Any product used daily on thin, frequently shaved skin has plenty of opportunity to cause trouble. Fragrance is the most common culprit, alcohol can sting on freshly shaved skin, and bicarbonate of soda suits some people and not others.

The darkness isn't the product staining anything. It's your skin responding to being irritated over and over. Our guide to underarm rashes and irritation goes through which ingredients are most likely to be behind it and how to work out which one is yours.

Hair shadow

This one deserves more attention than it usually gets, because a great many people are treating pigmentation they don't actually have.

Underarm hair is coarse and dark, and the follicle sits deep. When you shave, you take the hair off at the surface but the shaft below stays where it is, visible through skin that's thin and pale by comparison. The result is a grey or bluish cast that looks very much like discolouration.

There's a simple way to tell. If the darkness lifts after waxing, epilating or laser hair removal, it was shadow rather than pigment. Waxing pulls the whole shaft out, while a razor cuts it level with the surface and leaves the rest in place.

If that turns out to be what you're seeing, changing your hair removal method is usually the only approach that makes a noticeable difference, because no skincare product can remove the appearance of hair beneath the skin. It's a frustrating answer if you've spent money on brightening products, but it's a much easier problem to solve than pigment.

Acanthosis nigricans

Occasionally, dark underarms are a skin sign of something happening elsewhere in the body.

Acanthosis nigricans shows up as darkened skin with a thickened, velvety texture, usually symmetrical, appearing in body folds: the underarms, the back of the neck, the groin. The texture change is the distinguishing feature. It isn't only colour, and it tends to develop over months rather than appearing suddenly.

It's associated with insulin resistance, and it can also be linked to certain medications, hormonal conditions and, more rarely, other underlying health issues. It is not caused by deodorant, by hygiene, or by anything you've done to your skin.

If what you're seeing has that velvety, thickened quality, particularly in more than one body fold, that's worth a conversation with your GP rather than a new skincare routine.

Can natural deodorant cause dark underarms?

Sometimes, though not for the reason people usually assume.

The switch from antiperspirant to natural deodorant often coincides with darkening, and because deodorant is frequently the only thing anyone puts on their underarms, it gets the blame. There's also a persistent idea that natural formulas are inherently gentle and therefore couldn't possibly be the cause. Neither of those is quite right.

Any product can irritate skin, natural or otherwise. Essential oils are a reasonably common source of contact allergy. Bicarbonate of soda is alkaline where skin is mildly acidic, which plenty of people never notice and some react to. If a product irritates you daily, the pigment response follows, and that holds true whatever the ingredients list looks like.

The useful distinction isn't natural versus synthetic. It's whether a particular formula suits your particular skin.

There's a timing coincidence worth knowing about too. Switching away from an antiperspirant means you start sweating under your arms again, which brings more moisture and friction to an area that had been kept artificially dry. That on its own can make existing darkening more noticeable without anything new having caused it.

If your deodorant stings, itches or leaves the skin red, that's the thing to change, and a bicarb-free formula is where most people with reactive skin should start. Not because it will lighten anything, but because pigment can't settle while the skin is being irritated every morning.

How to improve dark underarm skin

A word on expectations first, because this is slower than most of the internet suggests. Post-inflammatory pigment fades over months rather than weeks, and only once the cause has stopped. Shadow doesn't fade at all, because there's nothing there to fade. And skin changes with a medical cause need addressing at the source.

Find the trigger and remove it. This matters more than anything you apply. Whatever is irritating the skin has to stop before the colour can improve.

Reconsider your hair removal. If shadow is the issue, waxing, epilating or laser will make a visible difference where nothing else has. If shaving is causing irritation, a fresh sharp blade, shaving in the evening rather than just before deodorant goes on, and a lighter hand all help.

Reduce friction where you can. Looser sleeves, softer fabrics, and no scrubbing at the area, which is exactly the sort of irritation that caused the problem in the first place.

Exfoliate gently, if at all. Light exfoliation can help with surface build-up, but underarm skin is thin and reactive. Anything that leaves it pink is doing more harm than good.

Keep the skin comfortable. Skin that isn't dry or tight recovers more readily from irritation. This isn't a brightening step, it's a barrier step, and it's the one most people skip.

Give it time. Several months is normal, and it fades gradually rather than all at once, so it's easy to miss the progress while you're looking at it every day.

When to see a doctor

Worth getting checked rather than treating at home if:

• The skin is thickened or velvety in texture, not simply darker
• The darkening appears in several body folds at once, such as the underarms and the back of the neck
• It has developed noticeably over a few months without any obvious cause
• There's itching, pain, a persistent rash or any bleeding
• You've removed every likely irritant and nothing has improved after several months

None of that means something is seriously wrong. It means the answer isn't in a skincare routine, and a GP can take a proper look in a few minutes.