What are the possible causes of a metallic taste on the tongue and how should it be evaluated?

Metallic Taste in the Tongue: Diagnostic Approach

Begin with a thorough medication review, as drug-induced dysgeusia is the most common reversible cause, with zopiclone/eszopiclone causing metallic taste in 7-26% of patients. 1

Initial Clinical Assessment

Medication History

• Zopiclone/eszopiclone is the single most common medication causing metallic taste (7-26% incidence), making it a primary consideration 1
• Review all medications systematically, as 17% of registered drugs can cause dysgeusia and 3.7% cause hypogeusia 2
• Local anesthetics can cause metallic taste as an early sign of systemic toxicity, accompanied by circumoral numbness 1
• Antineoplastic agents, systemic antiinfectives, and nervous system drugs are the most common drug categories causing taste disorders 2

Oral Examination Specifics

• Inspect for normal-appearing mucosa: If present with burning sensation and metallic taste, consider burning mouth syndrome (BMS), which predominantly affects peri- and postmenopausal women 3, 4
• Remove dentures and palpate floor of mouth to identify ulcers or masses 5
• Examine tongue appearance: Bright red tongue suggests BMS, candidiasis, or systemic conditions; black tongue suggests bacterial/fungal overgrowth 6, 7
• Document all dental restorations and implants: 10-15% of the population has metal hypersensitivity to dental alloys, which releases nickel, aluminum, vanadium, and titanium 3
• Check for periodontitis: Blood contamination from gum disease can cause metallic taste 3

Pertinent History Elements

• Age and gender: BMS is uncommon before age 30 (40 in men), typically occurs 3-12 years post-menopause in women 4
• Recent COVID-19 infection: Associated with taste disturbances including metallic taste 1
• Iodine exposure: Topical disinfectants, iodinated contrast agents, or environmental chemicals cause metallic taste as a cardinal symptom 3
• Cancer treatment history: Metallic taste occurs in 29% of cancer patients undergoing treatment 8

Recommended Blood Work

Order the following initial panel 1:

• Complete blood count with blood film: Screen for polycythemia vera, anemia, and leukemia 5, 1
• Comprehensive metabolic panel: Assess kidney and liver function for chronic kidney disease and aluminum toxicity 1
• Erythrocyte sedimentation rate (ESR): Screen for inflammatory conditions 1
• Serum zinc levels: Nutritional deficiency can cause taste disorders 3
• 24-hour urinary iodine excretion with thyroid function tests (TSH, free T4): If iodine toxicity suspected 3
• Serum aluminum levels: If chronic kidney disease present (baseline <20 μg/L; consider deferoxamine test if 60-200 μg/L) 1
• JAK2 V617F mutation analysis: If polycythemia vera suspected based on CBC findings 1

Blood Sampling Precautions 1

• Perform before meals
• No smoking or nicotine for 4 hours prior
• No alcohol for 12 hours prior
• Avoid dental work 1-2 hours before sampling
• Note recent acute infections (especially upper respiratory infections within 2 weeks)

Diagnostic Algorithm

If Medication-Related

• Discontinue or substitute the offending medication if clinically feasible 9, 2
• Note that lasting impairment may occur even after medication cessation 9

If Burning Mouth Syndrome Diagnosed

• Alpha-lipoic acid 200-600 mg daily in divided doses is first-line treatment 3
• Cognitive behavioral therapy is beneficial, as reassurance and education are crucial 3
• The condition typically does not worsen, and treatment is palliative 4

If Dental Metal Hypersensitivity Suspected

• Pre-implantation screening using skin patch testing or lymphocyte transformation assays for patients with metal intolerance history 3
• Consider removal or replacement of dental restorations releasing lead and cadmium into saliva 3

If Iodine Toxicity Suspected

• Measure 24-hour urinary iodine excretion combined with thyroid function tests 3
• Identify and eliminate iodine sources 3

If Hematological Disorder Found

• Refer to hematology for polycythemia vera management if JAK2 mutation positive 1
• Address anemia or leukemia based on bone marrow biopsy and immunotyping results 5

Critical Pitfalls to Avoid

• Do not dismiss normal-appearing oral mucosa: BMS is diagnosed specifically by the absence of visible abnormalities 3
• Do not attribute to psychiatric causes prematurely: Rule out nutritional deficiencies, metal exposure, and iodine toxicity first 3
• Do not overlook the underlying disease: The medical condition requiring medication (epilepsy, migraines, hypothyroidism, cancer) may be the actual culprit rather than the medication itself 9
• Do not assume reversibility: Chemosensory alterations may be permanent even after removing the offending agent 9

If Initial Workup Inconclusive

Refer for specialized taste testing using standardized methods such as "Sniffin' Sticks" or other validated olfactory/gustatory assessment tools 1

Consider neurological evaluation for persistent unexplained metallic taste, as simple partial seizures from amygdala, hippocampus, or parietal operculum can cause unpleasant chemosensory sensations 9