A biological or chemical substance or agent that has an adverse effect on the structure or function of the central and/or peripheral nervous system.()

Exposure to chemical substances can cause adverse effects on the nervous system (neurotoxicity). Chemicals toxic to the central nervous system can induce confusion, fatigue, irritability, and other behavioural changes. Exposure to methyl mercury and lead cause central nervous system toxicity, and can also cause degenerative diseases of the brain (encephalopathy). Chemicals toxic to the peripheral nervous system affect how nerves carry sensory information and motor impulses from the brain to the rest of the body.