It could be due to infection of gums or cavities in teeth
. It could also be an abnormal perception of bitter taste
called dysgeusia. GERD and heart burn too cause metallic taste in mouth. Post nasal drip as in sinusitis, salivary glands infection, pharyngitis or strep infection too are common causes. Certain dentures and artificial tooth or filling of dental cavities too give a metallic taste. Sjogren’s syndrome and copper or lead toxicity are other possibilities.