Abnormal ocular movement (e.g. nystagmus) can often be observed in select CNS pathology.

Certain drugs/toxin overdose can also induce nystagmus.

• Anti-epileptics: carbamazepine, lamotrigine, topiramate, phenytoin
• Ethanol
• Ketamine, phencyclidine (PCP), dextromethorphan – vertical or rotary nystagmus
• Serotonergic syndrome/5-HT agonists – opsoclonus
• Monoamine oxidase inhibitors – ping-pong nystagmus
• Lithium
• Scorpion envenomation 

In an "unknown" intoxication, physical exam findings such as nystagmus may help narrow the identity of the suspected ingestion/overdose.