Is that dark spot decay — or just a stain?

Colour alone does not decide it. The most reliable clue is not the shade but the pattern: staining is usually even-handed, appearing on many teeth at once and often symmetrically on both sides, while decay is a local event on one tooth in one spot. The second clue is texture — a stain is hard and smooth, a cavity is soft and catches. Brown is not automatically decay, and decay is not automatically brown: early decay is chalky white, and a dark spot that has hardened and stopped may need nothing more than watching. What genuinely settles the question is a dried tooth, gentle probing and, where the mark sits between two teeth, a small X-ray. Anything on a single tooth that has appeared or grown recently is worth showing to a dentist.

Run these before you worry.

1. One tooth, or many? Discolouration on many teeth at once — a general yellowing, a brownish film along the gum line, dark grooves across several molars — almost always points to surface staining, ageing enamel or something systemic. Decay does not arrive on eight teeth in the same week.

2. Is it symmetrical? Look at the matching tooth on the other side. Marks that mirror each other on left and right usually come from how the teeth were formed or from what passes over them daily. Decay is indifferent to symmetry; it appears where plaque sits undisturbed, which is rarely mirror-image.

3. Where exactly is it? Stains collect where surfaces are rough or sheltered: the grooves of chewing surfaces, the gum line, the inner surfaces of the lower front teeth. Decay favours the same places, so location alone will not decide it — but a mark in the middle of a smooth, flat surface is unusual for staining.

4. Has it changed? Stains build up slowly and can improve after a cleaning. A mark that has appeared over a few months, spread, or developed a shadow underneath the enamel is behaving like decay.

If your answers are “many teeth, symmetrical, in the usual places, unchanged for years”, you are almost certainly looking at discolouration. If they are “one tooth, one spot, and it wasn’t there last year”, book an appointment.

These come off, and they are the commonest reason people worry about a dark tooth.

Food and drink. Coffee, black tea, red wine, cola, curry and tomato-based sauces all deposit pigment into the thin protein film that forms on enamel within minutes of cleaning. Tea is a stronger stainer than coffee for many people.

Tobacco. Produces a characteristic yellow-brown to almost black film, heaviest on the inner surfaces of the lower front teeth and around the gum line.

Chlorhexidine mouthwash. Highly effective against bacteria, and well known for producing a brownish discolouration when used for weeks rather than days. It is a documented and reversible side effect, not damage.

Iron supplements and black stain. Liquid iron can leave dark deposits, which is relevant in pregnancy and for children on drops. Black stain — a thin dark line following the gum line in some children, produced by pigment-forming bacteria — looks alarming and is harmless; it polishes off and tends to return.

These do not polish off, because the pigment is in the tooth structure rather than on it.

Age. Enamel thins over the decades while the dentine beneath it grows thicker and more yellow. The result is a gradual, even warming of colour across the whole dentition — the single most common cause of teeth people describe as “duller than they used to be”.

Enamel formation. Dental fluorosis produces white flecks or, in more marked cases, brown mottling, typically symmetrical and present since the teeth came through. Illness or certain medicines during tooth development — tetracycline antibiotics being the classic example — can leave banded discolouration in the same symmetrical pattern.

Old fillings and metal. A greyish shadow spreading from beneath an old filling is often the restoration showing through thinning enamel rather than decay underneath it. Only an examination distinguishes the two reliably, since decay under a filling looks much the same from above.

Four patterns deserve an appointment rather than a whitening toothpaste.

A single tooth turning grey or brown from within. Especially after a knock, even one from years ago. This is usually about the nerve rather than the surface, and the appropriate response is an examination — whitening will not touch it, and the underlying situation may need treating.

Chalky white patches, particularly at the gum line or beside orthodontic brackets. Counter-intuitively, white can be the more urgent colour: these are demineralised areas, the earliest visible stage of decay, and the one stage that can often be stopped without drilling.

A brown or yellowish softening right at the gum line on an exposed root. Root surfaces have no enamel, and decay there develops more readily than on a crown surface. It is easy to mistake for ordinary staining because it sits exactly where staining also collects.

A dark line or a step at the edge of an existing filling. Sometimes stain in a microscopic gap, sometimes a failing margin with decay behind it. Under a microscope the difference is usually visible; from a bathroom mirror it is not.

If there is one situation where a confident home judgement is unwise, it is this one.

The chewing surfaces of molars carry deep, narrow grooves. Pigment settles into them, so dark grooves are extremely common and mostly harmless. But those same grooves are a favourite starting point for decay, and an early lesion there can look identical from above while being considerably wider underneath — decay travels more freely once it passes the enamel and reaches the softer layer below.

This is why dentists dry the tooth, probe gently rather than forcefully, and use a small X-ray when there is genuine doubt. It is also why sealing the grooves of newly erupted permanent molars in children is standard preventive practice: it removes the ambiguity before it arises.

Different problems, different answers — and the mismatch between them is where money gets wasted.

Two practical notes. Whitening does not change the colour of fillings, crowns or veneers, which is why existing restorations in visible positions often need renewing afterwards to match — the whitening article covers this in detail: Sensitive teeth after whitening. And a professional cleaning is generally a private service, though some statutory funds contribute; ask what applies to you before booking rather than after.

You are pregnant. Iron supplements, morning sickness and gums that react more strongly all change the colour picture. Cleanings and examinations are entirely appropriate during pregnancy; elective whitening is normally deferred.

Your child has dark marks. Black stain along the gum line is harmless pigment. White or brown marks on the upper front teeth, at the gum line, are the classic early sign of decay in small children and should be seen — Tooth decay in children: when to see a dentist covers what to look for.

You wear braces or aligners. White patches around bracket edges are demineralisation, not staining, and the response is fluoride and technique rather than polishing. Check after removing an aligner, not through it.

You are planning whitening. Any sensible sequence starts with an examination, because whitening an undiagnosed cavity is unproductive and existing fillings in the smile line need to be part of the plan from the start.

You are new to Germany. “Is this a cavity or a stain?” is a hard question to pose in a second language, and the answer matters financially: one is treatment, the other is largely cosmetic. You can ask it in English here. English-Speaking Dentist in NRW explains how examinations, cover and appointments work for people who have recently arrived in Wuppertal and the surrounding area.