Black or dark discoloration on the sides of your tongue is usually caused by something harmless: a buildup of dead cells, a reaction to something you ate or drank, or a side effect of medication. Less commonly, it can signal a dental issue or an underlying health condition worth investigating. The cause often depends on whether the discoloration appeared suddenly or developed over time, and whether it’s on the surface or seems embedded in the tissue.
Black Hairy Tongue
The most well-known cause of tongue darkening is a condition called black hairy tongue. It happens when the small bumps on the tongue’s surface (papillae) grow longer than usual and trap bacteria, food particles, and dead cells. The result looks dark, sometimes almost furry. Black hairy tongue most commonly appears on the top of the tongue toward the back, but the discoloration can spread to the sides, especially if oral hygiene is poor or the tongue isn’t being cleaned regularly.
Several things raise your risk: taking antibiotics (which shift the balance of bacteria and yeast in your mouth), smoking, drinking a lot of coffee or black tea, heavy alcohol use, chronic dry mouth, and eating mostly soft foods that don’t naturally scrub the tongue’s surface. Mouthwashes containing oxidizing agents like peroxide can also irritate the tongue and contribute. People with weakened immune systems, including those with HIV or undergoing cancer treatment, are more susceptible.
The good news is that black hairy tongue is almost always reversible. Regular brushing of the tongue, quitting tobacco, switching mouthwashes, staying hydrated, and eating a diet with more whole fruits and vegetables typically clears it up. If the discoloration doesn’t improve within a couple of weeks of better oral care, it’s worth getting checked.
Medications That Stain the Tongue
If you recently started a new medication and noticed your tongue turning dark, there’s a good chance the two are connected. The most common culprit people encounter is bismuth subsalicylate, the active ingredient in Pepto-Bismol. Bismuth reacts with sulfur naturally present in your saliva to form a black compound called bismuth sulfide. This can coat the tongue (including the sides and edges) and even darken your stool. It’s temporary and clears once you stop taking the medication.
Several prescription drug classes can also cause tongue or oral pigmentation. Antibiotics in the tetracycline family, particularly minocycline, are well documented to cause dark staining in the mouth, skin, and nails. Antimalarial drugs like chloroquine and hydroxychloroquine (also prescribed for autoimmune conditions like lupus) can produce a blue-gray to blue-black discoloration across the tongue, gums, and palate. Certain chemotherapy drugs, antipsychotics in the phenothiazine class, oral contraceptives, and some HIV medications have all been linked to oral pigmentation changes. Some of these medications cause surface staining that fades after discontinuation, while others trigger actual melanin production in the tissue, which can be longer-lasting.
If you suspect a medication is involved, don’t stop taking it on your own. Bring the discoloration to your prescriber’s attention so they can evaluate whether the benefit of the medication outweighs this side effect.
Mouthwash and Topical Products
Chlorhexidine, a prescription-strength antibacterial mouthwash commonly used after dental procedures or for gum disease, is known to stain the tongue and teeth. The discoloration can appear brown, gray, or black and tends to concentrate wherever the rinse makes the most contact, which often includes the sides of the tongue. The staining isn’t permanent and fades after you stop using the product, though it can take some time to fully clear.
Amalgam Tattoos From Dental Work
If the dark spot on the side of your tongue looks like a flat, grayish-blue or black mark and you’ve had dental work done, it could be an amalgam tattoo. This happens when tiny fragments of silver amalgam filling material get embedded in the soft tissue of the mouth during a dental procedure. The result is a flat, painless spot that can have well-defined or slightly irregular borders. It doesn’t change over time (though slight enlargement is possible), and it’s completely benign.
Amalgam tattoos are one of the most common causes of localized dark spots inside the mouth. Because the sides of the tongue sit right next to the teeth and fillings, they’re a natural location for this kind of accidental pigment deposit. No treatment is needed, but if the spot is new or you’re unsure of its origin, a dentist can often identify it on an X-ray, since the metal particles show up clearly.
Adrenal Insufficiency and Hormonal Causes
Rarely, dark patches inside the mouth point to something systemic. Addison’s disease, a condition where the adrenal glands don’t produce enough cortisol, causes widespread hyperpigmentation that often shows up inside the mouth before it appears on the skin. The darkening can affect the tongue, inner cheeks, gums, and lips. It happens because the pituitary gland overproduces a hormone that stimulates melanin-producing cells as it tries to compensate for low cortisol.
Oral pigmentation from Addison’s disease doesn’t appear in isolation. You’d typically also notice fatigue, muscle weakness, salt cravings, low blood pressure, and darkening of skin creases (especially the palms). If dark spots in your mouth are accompanied by these symptoms, blood tests measuring cortisol and related hormones can confirm or rule out the diagnosis.
When Dark Spots Need Closer Attention
Most causes of a dark tongue are benign, but oral melanoma, while rare, does occur. It can appear as a pigmented spot, nodule, or plaque on the tongue, and the lateral border (side edge) of the tongue is one of the areas where oral cancers tend to develop. What makes oral melanoma tricky is that up to 25% of cases can look like benign lesions, and about 18% lack dark pigmentation entirely, appearing as red or flesh-colored growths instead.
Features that warrant a prompt evaluation include a spot that is growing, changing shape, bleeding, or ulcerating. A dark patch that appeared recently without any clear explanation (no new medications, no dietary changes, no dental work) also deserves a professional look. A dentist or oral medicine specialist can perform a biopsy if there’s any uncertainty. Early detection makes a significant difference in outcomes for the small number of cases that do turn out to be serious.
Simple Steps to Clear Surface Staining
If the discoloration is on the surface rather than embedded in the tissue, you can usually resolve it at home. Brush your tongue gently twice a day with a soft toothbrush or use a tongue scraper. Stay well hydrated to prevent dry mouth, which allows bacteria and debris to accumulate. Cut back on coffee, black tea, and tobacco if those are part of your routine. Switch away from mouthwashes that contain peroxide or other oxidizing agents. Eating crunchy, fibrous foods like apples, carrots, and celery helps naturally scrub the tongue’s surface.
If the dark coloring doesn’t respond to improved hygiene within two to three weeks, or if it’s a single well-defined spot rather than a diffuse stain, have it looked at by a dentist or doctor who can determine whether it’s something that needs further evaluation.