Metallic taste

Common causes include:

• gingivitis and/or periodontitis - in these cases pus is slowly seeping out from the gums giving rise to a sometimes metallic or bitter taste. Patients may also complain (or exhibit) halitosis.
• drugs which may cause a metallic taste include: (1)
  • metronidazole
  • gold
  • metformin
  • ACE inhibitors
  • terbinafine
  • zopiclone
  • lithium
  • chemotherapy.
• idiopathic
• post-viral taste disturbance

Also consider: (2)

• respiratory infections of the upper respiratory tract - especially of the tonsils and sinuses - can leave an unpleasant taste in the mouth
• gastric reflux - this may cause bitterness within the mouth
• oral dryness e.g. in Sjogren's syndrome - may lead to bitter taste or generalised loss of taste

If there is no oral cause such as gingival inflammation or systemic cause (e.g. drug therapy) to explain the patient's symptom then review the patient's social history to determine if there are any lifestyle stresses.

Reference:

1. Naik BS, Shetty N, Maben EVS. Drug-induced taste disorders. Eur J Intern Med. 2010;21(3):240-243.
2. Liu G, Zong G, Doty RL, Sun Q. Prevalence and risk factors of taste and smell impairment in a nationwide representative sample of the US population: a cross-sectional study. BMJ Open. 2016 Nov 9. 6 (11)