Amnesia

Amnesia is the (usually temporary) loss of memory. Amnesia is classified as either functional amnesia, caused by psychological factors, and organic amnesia, caused by illness or traumatic injury to the brain, or by certain chemical agents.

On at least three instances, The Doctor suffered from partial amnesia after regenerating. (DW: Castrovalva, Time and the Rani, Doctor Who). Except in the case of the regeneration into his eighth form (caused by anesthesia), it is unclear whether this should be classified as functional or organic amnesia; in other words, it could be due to the biological process of regenerating, or the psychological trauma of "dying" and being reborn.

Another common cause of amnesia seems to be forcibly transmatting people through time and space.

Instances of Organic Amnesia

• The First Doctor, Ian Chesterton, Barbara Wright and Susan Foreman briefly suffered from amnesia after the TARDIS's Fast Return Switch got stuck. (DW: The Edge of Destruction)

• The First Doctor force-fed an amnesia pill to William Shakespeare. (MA: The Empire of Glass)

• During the Doctor's exile on Earth, the Time Lords suppressed his memories of how the TARDIS worked as part of his punishment. These were later restored to him. (DW: The War Games, The Three Doctors)

• The victims of an alien Extractor suffered from permanent amnesia. Liz Shaw suffered from temporary amnesia from being a car accident after escaping the Extractor. (DWA: The Mind Extractors)

• The Sixth Doctor suffered from partial amnesia after being transported to Space Station Zenobia for his trial. (DW: The Mysterious Planet)

• Ace suffered from amnesia when the Doctor accidentally erased her memory while editing his own. (NA: Timewyrm: Genesys)

• The Ninth Doctor suffered from partial amnesia after being transported to Satellite Five. (DW: Bad Wolf)

• The staff of Torchwood 3 often employed a drug called retcon to induce amnesia. (TW: Everything Changes)

• The Panjistri supplied the Kirithons with zavát which, in addition to being their main food source, weakened their resistance to telepathic suggestion, allowing the Panjistri to suppress certain memories. (NA: Timewyrm: Apocalypse)

Instances of Functional Amnesia

• The victims of the War Lords suffered from post-hypnotic amnesia. (DW: The War Games)

• Alistair Gordon Lethbridge-Stewart suffered from amnesia from 1977 to 1983 as a result of meeting his future self. (DW: Mawdryn Undead)

While likely caused by the psychological trauma of meeting himself, it may also be organic amnesia caused by the release of energy when the two Brigadiers touched.

• The Doctor suffered from amnesia after being forced to destroy Gallifrey. (EDA: The Ancestor Cell)

Other instances of Amnesia

• The Fourth Doctor suffered from amnesia after being transported to Deepcity. (MA: A Device of Death)
• The Eighth Doctor suffered from amnesia after falling into the Master's trap. (EDA: The Eight Doctors)
• The Eighth Doctor suffered from amnesia again while on Espero. (EDA: Halflife)
• Chris Cwej suffered from amnesia while on Ardethe. (BNA: Deadfall)