Vitamin B-12 deficiency is a known cause of central and peripheral nervous system damage. It has been implicated in sensory neuropathy and autonomic nervous system dysfunction -- which can in turn have a role in chronic, unexplained coughs.
A recent study showed that vitamin B-12 deficiency patients had a higher prevalence of laryngeal hyperresponsiveness. After being given B-12 supplements, their symptoms and laryngeal, bronchial, and cough thresholds significantly improved.
According to the study in the American Journal of Clinical Nutrition:
"This study suggests that [vitamin B-12 deficiency] may contribute to chronic cough by favoring sensory neuropathy as indicated by laryngeal hyperresponsiveness and increased NGF expression in pharyngeal biopsies of [vitamin B-12 deficiency] patients. [Vitamin B-12 deficiency] should be considered among factors that sustain chronic cough, particularly when cough triggers cannot be identified."