Isoniazid toxicity occurs in populations at risk, including patients with chronic kidney failure or NAT2 polymorphisms, who have a disturbed metabolism of pyridoxine or isoniazid, respectively, and those on renal replacement therapies, in whom pyridoxal phosphate – the active metabolite of pyridoxine – is inadvertently removed by dialysis. The gene discussed is NAT2; the disease is chronic renal failure syndrome.