What are the causes of dysgeusia (sour or metallic taste) in the mouth?

Causes of Sour or Metallic Taste in the Mouth

The most common causes of dysgeusia (sour or metallic taste) are burning mouth syndrome, medication effects, gastroesophageal reflux disease, dental metal exposure, nutritional deficiencies (particularly zinc), and age-related changes in taste perception. 1, 2, 3

Primary Etiologic Categories

Burning Mouth Syndrome (BMS)

• BMS is the most common cause of persistent metallic taste without medication exposure, characterized by burning sensations of the tongue and oral mucosa with altered (metallic) taste or diminished taste sensations. 1
• The condition presents with both positive symptoms (burning pain, dysgeusia, dysesthesia) and negative symptoms (loss of taste, paresthesia) involving the lips and tongue, mainly the tip and anterior two-thirds. 3
• BMS patients report persistently altered metallic taste or diminished taste sensations, with acidic foods such as tomatoes and orange juice causing considerable distress. 3
• The disorder is very uncommon before age 30 (age 40 for men), with onset in women typically occurring 3-12 years after menopause. 3
• Tongue biopsies show significantly lower density of epithelial nerve fibers in BMS patients compared to controls, supporting the concept that BMS is a small fiber neuropathic disorder with altered sensory processing. 3

Gastroesophageal Reflux Disease (GERD)

• GERD can create obstinate sour-taste sensations through increased oral sensitivity combined with acid reflux into the oropharynx. 4
• Patients typically report that sour-tasting meals worsen the sensation, accompanied by eructation and regurgitation. 4
• Upper GI endoscopy may reveal duodenal erosion, superficial gastritis, and erosive esophagitis in affected patients. 4

Dental Metal Exposure and Hypersensitivity

• Approximately 10-15% of the general population exhibit hypersensitivity to one or more metals commonly used in dental implants, which manifests as metallic taste. 1
• Metal hypersensitivity shows significantly higher incidence in females and can arise at any age through chronic low-level exposure to metal ions from dental restorations. 1
• Trace elements such as nickel, aluminum, vanadium, and titanium released from dental work provoke allergic reactions presenting with metallic taste. 1
• Dental alloy restorations can release lead and cadmium into saliva, contributing to dysgeusia. 1

Nutritional Deficiencies

• Zinc deficiency is a critical determinant for correct integration of the taste system at multiple levels, including synthesis of gustin (a parotid metalloprotein secreted into saliva) and neurotransmission of gustatory signals. 5
• Serum and urinary zinc levels should be measured using flameless atomic absorption spectrophotometry in dysgeusic patients. 5
• Quantification of leucocyte alkaline phosphatase activity (a zinc metalloenzyme) provides useful diagnostic information. 5

Age-Related Physiological Changes

• Healthy aging causes alterations in olfaction and gustatory sensation, with changes in muscles of mastication and lower salivary flow rates exacerbating taste problems. 1, 2
• Age-related sarcopenia reduces oral tongue muscle strength used for swallowing, diminishing taste stimulus delivery and contributing to metallic taste perception. 1
• Decreased salivary flow rates in elderly patients, combined with altered mastication-muscle function, lower oral clearance and raise salivary pH, creating an environment predisposing to metallic taste. 1

Xerostomia (Dry Mouth)

• Dry mouth from sicca syndrome presents with abrupt onset, usually without dry eyes, and is distinct from Sjögren's syndrome. 6, 2
• Sicca syndrome occurs in 2-11% of patients receiving immune checkpoint inhibitor (ICI) treatment, typically within 3 months of therapy initiation. 6
• Sjögren's syndrome affects approximately 0.4% of the population with a female-to-male ratio of 20:1, characterized by lymphocytic infiltration of lacrimal and salivary glands. 2
• Medications with anticholinergic effects (tricyclic antidepressants, antihistamines), beta-blockers, SSRIs, opioids, and stimulants commonly cause dry mouth contributing to taste alterations. 2

Iodine Toxicity

• Excess iodine from topical disinfectants, iodinated contrast agents, or environmental chemicals causes metallic taste as a cardinal symptom. 1
• Diagnostic evaluation requires measuring 24-hour urinary iodine excretion combined with thyroid function tests (TSH, free T4). 1

Immunotherapy-Related Oral Dysesthesia

• Oral dysesthesia from immune checkpoint inhibitors is described as oral pain with "burning" sensation in the absence of, or disproportionate to, skin changes, oral sensitivity, dysgeusia, or other altered sensation with normal clinical findings. 6
• The prevalence of ICI-related oral/oropharyngeal pain is estimated at 4%. 6

Diagnostic Approach

Initial Clinical Assessment

• Perform detailed oral examination to assess for BMS (normal-appearing mucosa), dental restorations, gum disease, or blood contamination from periodontitis. 1
• Document all dental restorations, implants, and recent dental work for clinical assessment. 1
• Rule out mimicking conditions such as candidiasis and dysphagia, which can present similarly. 2

Laboratory Evaluation

• Measure serum zinc, 24-hour urinary iodine, and thyroid function tests (TSH, free T4). 1
• Consider serum copper and ceruloplasmin if Wilson's disease is suspected. 1
• Assess liver function tests, B12 and folate levels, and serum protein electrophoresis, as many pathologic states of these organ systems relate to taste disorders. 5

Specialized Testing

• For individuals with history of metal intolerance, pre-implantation screening using skin patch testing or lymphocyte transformation assays identifies potential hypersensitivity before dental procedures. 1
• Measure whole salivary flow rates objectively before initiating treatment, as subjective sensation may not correlate with actual salivary output. 2
• Upper GI endoscopy should be performed when GERD is suspected based on associated symptoms of eructation, regurgitation, and worsening with acidic foods. 4

Management Strategies

For Burning Mouth Syndrome

• Alpha-lipoic acid 200-600 mg daily in divided doses is first-line treatment for BMS. 1
• Cognitive behavioral therapy is beneficial, as reassurance and education are crucial since the condition typically does not worsen. 1
• Gabapentin may be used for neuropathic pain management, as BMS is largely considered neuropathic in origin. 3

For GERD-Related Sour Taste

• Proton pump inhibitor therapy (lansoprazole 30 mg daily) can eliminate sour taste sensations within 6 weeks in GERD-related cases. 4
• Patients typically experience 70% subjective improvement after 2 weeks of PPI therapy. 4

For Immunotherapy-Related Dysesthesia

• Dietary modifications are recommended for all patients with oral dysesthesia, avoiding crunchy, spicy, acidic, or hot food/drink. 6
• For mild symptoms (Grade 1): Continue immunotherapy while assessing location and character of pain, ruling out odontogenic or mucosal sources. 6
• For moderate to severe symptoms (Grade 2-3): Hold immunotherapy and initiate gabapentin, high-potency topical steroids (gel or dental paste preferred), or viscous lidocaine. 6

For Sicca Syndrome

• Improve hydration and limit caffeine intake; use topical measures including water sips, saliva substitutes, and moisture-preserving mouth rinses. 6
• For moderate to severe dry mouth: Prednisone 20-40 mg daily for 2-4 weeks with taper, plus systemic sialagogues (cevimeline or pilocarpine). 6
• Rheumatology and dental referrals are essential, as severe sicca syndrome can result in dental caries and tooth loss if untreated. 6

For Metal Hypersensitivity

• Remove or replace offending dental restorations containing nickel, aluminum, vanadium, or titanium in confirmed cases of metal allergy. 1

For Nutritional Deficiencies

• Zinc supplementation should be initiated when deficiency is documented by serum and urinary levels. 5

Critical Pitfalls to Avoid

• Normal-appearing oral mucosa should not be dismissed as excluding pathology—BMS is diagnosed specifically by the absence of visible abnormalities. 1
• Avoid premature attribution to psychiatric causes without ruling out nutritional deficiencies, metal exposure, and iodine toxicity. 1
• Do not assume subjective dry mouth correlates with actual salivary output; objective measurement is required. 2
• In elderly patients with neurological disease, distinguish true xerostomia from dysphagia-related pooling, as management strategies differ when dryness is due to clearance deficits rather than reduced saliva production. 2
• Recognize that current BMS treatments are palliative only and may not be significantly better than credible placebo treatment, though few studies report relief without intervention. 3