Humans exposed to nicotine, smokers, and patients receiving nicotine treatments (patients with Alzheimer's disease, Parkinson's disease, or ulcerative colitis) may have altered CYP2E1 levels and activity, mediated metabolism of drugs and toxins, and altered toxicity generated by the CYP2E1 metabolism [44]. This evidence concerns the gene CYP2E1 and early-onset autosomal dominant Alzheimer disease.