The Underarm Guide

Why underarms get darker

It is pigment, not dirt. When skin is irritated over and over, it defends itself by producing extra melanin in that spot. That is hyperpigmentation, and it is the same process behind a dark mark left by a spot on your face.

Underarm skin is thinner than the skin on your arms, it gets shaved, and it spends the day under friction from sleeves and straps. That combination is close to ideal conditions for pigment to build up.

Which is why the usual instinct backfires. Scrubbing harder is more irritation. More irritation is more pigment.

The most common triggers

Shaving and hair removal. Friction from tight clothing. Irritation from deodorants and antiperspirants. Buildup of dead skin. Genetics and your natural skin tone decide how easily it happens to you, which is why two people with the same routine end up in different places.

Illustration of the character holding a razor, with a faint mark under the raised arm — The razor is the most repeated trigger. Not because shaving is bad — because a blunt blade on thin skin is irritation, and irritation is the whole mechanism.

What makes it worse

These come up constantly online. Every one of them is an irritant, and irritation is the cause.

Lemon juice. Acidic, and it makes skin more sun-sensitive. It does not lighten pigment, it inflames the area.
Baking soda. Strongly alkaline. It strips the barrier that is already damaged.
Hydrogen peroxide. A wound antiseptic, not a skincare active.
Coarse scrubs and loofahs. Physical abrasion on skin that is thin to begin with.
Rubbing hard in the shower. Same problem, done daily.
Dry shaving, or a blunt blade. The single most repeated trigger.
Switching deodorant over and over. The deodorant is usually not the cause, so this rarely changes anything — and each new formula is a fresh chance to react.

Illustration of the character holding half a lemon — Lemon

Illustration of the character with white paste under one arm beside a bowl of powder — Baking soda

Illustration of the character scrubbing one underarm hard with an abrasive pad — Scrubbing

Illustration of the character sitting calmly with the lemon, powder and scrubbing pad set aside — Stopping

The pattern is simple. Anything that irritates the skin adds pigment instead of removing it.

One email, once

We’ll write to you in week three.

Week three is where almost everyone quits. You are far enough in that you expected to see something, and it is usually just before the change shows up. So we send one email at exactly that point to tell you that what you are seeing is normal — plus the short checklist of the four things people get wrong without noticing.

One email in week three and the checklist. Nothing else unless you ask for it, and you can unsubscribe from the first one.

How to use undim

Clean and dry

It has to be dry or it will not absorb properly. After a shower, give it a few minutes.

A pea-sized amount

More is not faster. Massage until it disappears — about ten seconds per side. There should be no residue and nothing sticky.

Morning and night, every day

This is the one that decides the outcome. Twice a day, without gaps. One tube is thirty days at that rate.

If you shave

Not straight after. Shaving leaves micro-damage, and an active on top of that can sting and add the exact irritation you are trying to stop. The simplest rule: shave at night, apply undim in the morning — or just leave a few hours between the two. After waxing or hair removal cream, give the skin a full day.

Illustration of the character holding a plain cream tube against its body — A pea-sized amount, twice a day. The amount is not what decides the result — not skipping days is.

Where you can use it

Underarms, elbows, knees and bikini line. Not the face — it is formulated for thicker body skin.

What happens, week by week

Weeks 1–2

Niacinamide starts working on the pigment underneath. What you notice first is that the skin feels softer. Tone has not moved yet, and it is not supposed to have.

Weeks 3–4

The first real change in tone. You usually notice it before anyone else does — and often it shows in a photo before it shows in the mirror, because you see your own skin every day.

Weeks 6–8

Where it settles into the full result. Eight weeks is the point of doing this. Two was never the finish line.

After

Two or three times a week holds it. It stops being a project and becomes part of the routine.

Illustration of the character lying face down, resting — Week three, in one picture.

Week three is where most people stop. It is far enough in that you expected to see something, and usually just before the change shows up. If you get there and feel like nothing is happening, that is not a sign it is failing. That is what week three feels like.

It is also the one moment worth putting a reminder on. We can email you then.

When it is not just pigmentation

If the darkened skin is also thickened and velvety to the touch rather than simply darker in colour, and especially if you also see it on your neck or groin, that can be acanthosis nigricans. It is associated with insulin resistance.

That is a medical sign, not a cosmetic one, and no brightening cream is the right answer for it. See a doctor first. We would rather tell you that than sell you something.

Before you give up, write to us

If you are past week four and genuinely seeing nothing, email hello@getundim.com and tell us how you have been using it. Most of the time something small is off — applying to damp skin, once a day instead of twice, or straight after shaving.

Illustration of the character sitting next to a sealed cardboard box, one arm resting on it — You keep the tube either way. There is nothing to pack up and send back.

And if it turns out this is not working for you, the thirty-day guarantee is real. No forms, no hoops, and you do not send the tube back half-used. You are risking an email.

You didn’t do anything wrong.