The mechanisms that cause amnesia are only partially understood, but the condition itself usually results from damage to the brain. The temporal and frontal lobes of the brain are primarily responsible for storing and retrieving information. However, memory is interwoven with many different brain functions and damage to virtually any part of the brain can trigger amnesia.

The brain can be damaged in many ways. For example, amnesia may occur following a severe blow to the head, a condition known as a traumatic brain injury. In some cases, these blows are severe and can lead to medical conditions such as coma. However, a lesser blow to the head that does not cause loss of consciousness can also trigger symptoms of amnesia. Car crashes, bicycle accidents, falls and sports mishaps all can result in blows to the head that may cause amnesia.

Many different conditions can also contribute to amnesia, especially those associated with delirium and/or dementia. Conditions that may contribute to amnesia include Alzheimer’s disease, tumors or infection in the brain, herpes encephalitis (inflammation of the brain caused by herpes simplex 1 virus), strokes or transient ischemic attacks (temporary reductions in flow of blood to the brain) and neurodegenerative illness.

Drug and alcohol abuse can lead to brain injuries that also cause amnesia. In particular, hallucinogens such as LSD or PCP can alter brain chemicals, making it more difficult for the person to recall memories. Certain legal drugs – including drugs used in general anesthesia, barbiturates and benzodiazepines – can also cause amnesia. Alcoholism may result in a condition called Wernicke-Korsakoff syndrome, which is characterized by memory loss.

In rare instances, Dissociative disorders (e.g., Dissociative amnesia, Dissociative identity disorder) may cause people to lose their memory. These disorders occur when people frequently escape reality by repressing their memories or taking on alternate identities, and do not involve medical trauma to the brain.

The aging process can cause some symptoms associated with amnesia, such as difficulty learning new materials and greater difficulty in recalling learned material. However, aging itself should not lead to significant amnesia.

Other sources of amnesia include severe emotional trauma and seizures.