Black or very dark teeth in adults usually come down to one of a handful causes: advanced decay, a dead nerve inside the tooth, tartar buildup, old dental fillings, certain medications, or a type of bacterial stain that looks alarming but is actually harmless. Some of these are surface-level problems you can polish away, while others signal damage deep inside the tooth. Knowing which category your dark tooth falls into determines what, if anything, needs to be done about it.
Tooth Decay
The most common reason a tooth turns black is untreated decay. A cavity starts as a small white or brown spot where acid from bacteria has dissolved the outer enamel. If left alone, the decay spreads deeper into the softer layer underneath (dentin), and the damaged tooth structure gradually darkens to deep brown or black. At that point the cavity is usually visible as a hole or a soft, crumbling area rather than a smooth surface.
The key distinction is that decay involves actual destruction of tooth structure. A dentist can tell the difference between a dark stain sitting on an intact surface and a cavity eating into it. Stains can be scraped or polished off. Decay cannot, because the mineral structure of the enamel or dentin has already broken down irreversibly.
A Dead or Dying Nerve
A tooth that has suffered trauma, a deep cavity, or even heavy orthodontic force can lose its blood supply. When the nerve tissue inside the tooth dies, blood cells break down and their pigments seep into the tiny tubes that make up the dentin layer. The tooth gradually shifts from its normal shade to gray, dark brown, or black.
This color change doesn’t happen overnight. The longer the breakdown products sit inside the tooth, the darker it gets. A tooth that was injured years ago may look noticeably darker than surrounding teeth, while one that was hit recently might only show a subtle grayish tint at first. A dead tooth won’t always hurt. Some people only discover the problem when they notice the color change in the mirror. Others develop an abscess at the root tip, which can cause swelling or a dull ache that comes and goes.
Old Silver Fillings
Silver amalgam fillings, widely used from the mid-1900s onward, contain a mix of metals including silver and mercury. Over decades, these metals corrode and release ions that penetrate into the surrounding dentin. Those ions form dark metallic sulfide compounds inside the tooth structure, staining it from the inside out. The result is a grayish-black shadow that shows through the enamel, sometimes making the entire tooth look dark even though the filling itself is intact.
This staining is cosmetic rather than harmful to the tooth’s health. But because the discoloration is embedded in the dentin, simply replacing the old filling with a tooth-colored one won’t fully eliminate it. The remaining dark dentin may still show through unless it’s covered with a crown or veneer.
Black Stain From Bacteria
Some adults develop a distinctive line of tiny black dots along the gumline of multiple teeth. This is called black stain, and it’s a specific type of bacterial buildup dominated by a group of bacteria that produce hydrogen sulfide. That hydrogen sulfide reacts with iron naturally present in saliva to form ferric sulfide, an insoluble black compound that bonds firmly to the tooth surface.
Black stain looks different from a cavity. It forms a dotted or continuous dark line that follows the contour of the gum, typically on the outer and inner surfaces of teeth rather than between them. It sits on top of intact enamel and can be removed with professional cleaning, though it tends to come back. Interestingly, research has found that people with this type of stain often have lower rates of cavities, possibly because the bacterial community responsible for the stain competes with cavity-causing bacteria.
Tartar Below the Gumline
Tartar (calculus) that forms above the gumline is usually yellowish or off-white. But tartar that develops below the gumline, in the pocket between tooth and gum, picks up blood from inflamed gum tissue. That blood mixes with the mineral deposit and turns it dark brown or black. When gums recede or when the tartar builds up enough to become visible at the gum margin, it can make the base of a tooth look black.
This type of discoloration is a sign of gum disease. The tartar irritates the gums, the gums bleed, and the blood darkens the tartar further in a self-reinforcing cycle. A dentist or hygienist removes it with scaling instruments, sometimes requiring a deeper cleaning under local anesthetic if the deposits extend far below the gumline.
Medications and Supplements
Liquid iron supplements are a well-known cause of dark tooth staining. Free iron ions from the supplement interact with compounds in your saliva and oral bacteria to form dark-colored deposits on the enamel surface. The acidic nature of most iron syrups also roughens the enamel slightly, giving the stain more texture to grip onto. The staining tends to concentrate near the gumline and between front teeth.
Chlorhexidine, an antibacterial mouthwash often prescribed after dental procedures or for gum disease, causes brown-to-black staining with regular use. The mechanism is similar: the chlorhexidine molecule binds to proteins on the tooth surface and reacts with dietary compounds (especially tannins in tea, coffee, and red wine) to produce dark pigments. The staining is extrinsic, meaning it can be polished off, but it returns as long as you keep using the rinse.
Tetracycline antibiotics taken during tooth development cause a different kind of staining. The drug incorporates directly into the tooth structure, producing gray, brown, or blue-black bands. In adults whose teeth are fully formed, tetracycline won’t cause new staining, but adults who were exposed as children may still carry those marks.
Extrinsic vs. Intrinsic Discoloration
Understanding whether the blackness is on the surface or inside the tooth matters because it determines your treatment options. External discoloration happens when pigments deposit on the outside of the tooth or in the thin protein film that coats enamel. Coffee, tobacco, iron supplements, chlorhexidine, and bacterial black stain all fall into this category. A professional cleaning can remove most of it, and whitening products can address residual surface staining.
Intrinsic discoloration comes from changes inside the enamel or dentin itself. A dead nerve, old amalgam fillings, tetracycline exposure, and genetic conditions affecting enamel formation all produce color changes that no amount of surface polishing will fix. These require either internal bleaching (placing a whitening agent inside a root-treated tooth), veneers, or crowns to restore a normal appearance.
How Dark Teeth Are Treated
For surface stains, professional cleaning with an ultrasonic scaler and polishing paste is usually enough. If the staining comes from iron supplements, switching to a different formulation or taking iron with a straw can reduce recurrence. For chlorhexidine staining, limiting tea and coffee intake during use helps, though the simplest solution is stopping the rinse once it’s no longer needed.
For a tooth darkened by a dead nerve, the standard path is root canal treatment followed by internal bleaching or a crown. Internal bleaching involves placing a whitening agent inside the hollow chamber of the tooth after the nerve has been removed. It’s effective for moderate discoloration but may need to be repeated.
Veneers, thin porcelain shells bonded to the front of a tooth, can mask severe discoloration. Research shows that even with heavily stained teeth, a veneer as thin as half a millimeter combined with a single bleaching session can effectively hide the dark color underneath. For the darkest cases, a full crown provides the most complete coverage. The choice depends on how much healthy tooth structure remains and how dark the underlying tooth is.
Teeth stained by old amalgam fillings present a unique challenge. Removing the filling eliminates the source of new staining, but the discoloration already embedded in the dentin stays. A composite (tooth-colored) filling can block some of the dark shadow from showing through, but a crown or veneer is often the more predictable cosmetic solution for front teeth.