This paper was a case report of a woman affected by chronic inflammatory demyelinating polyneuropathy (CIDP) and recessive myotonia congenita (MC) who was treated with mexiletine. The 44-year old female was reported to have gained benefit for CIDP symptoms and relapses following an intake of mexiletine (200 mg twice daily). The patients had a nearly complete remission of symptoms like muscle stiffness and weakness at up to three years after taking the therapy. She then developed an allergic reaction related to the drug, and it was withdrawn. Her condition worsened significantly after the medication was withdrawn regarding both CIDP and MC symptoms. “This is the first report on the association of CIDP and MC in the same patient. Such diseases may share some clinical symptoms related to a persistent sodium currents increase, which may be due either to the over-expression of sodium channels following axonal damage due to demyelination or to the chloride channel genes mutations.”