Neurotoxins are substances which can damage the nervous system. These may both be synthetic or naturally occurring and absorbed by the body through ingestion, inhalation, injection, or skin contact. The effects may be merely temporary or permanent.

For instance, a person may present with uncoordinated movements after snorting cocaine while a patient exposed to lead may suffer from irreversible damage. Unfortunately, the food that we often consume contain neurotoxins such as aspartame (artificial sweetener), diacetyl (found in the artificial butter of microwavable popcorn), monosodium glutamate, and mercury (in long-lived fish like tuna, swordfish, and orange roughy). Some of the symptoms of neurotoxicity include weakness or tingling in the limbs or paralysis, headache, vision problems, memory loss, depression, sexual dysfunction, and other cognitive and behavioral problems. Also, some venomous snakes, arthropods (spiders and scorpions) and mollusks (i.e. blue ring octopus) use their defense mechanisms.