Amnesia

Amnesia was the (usually temporary) loss of memory. Amnesia was classified as either functional amnesia, caused by psychological factors, or 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. (TV: Castrovalva, Time and the Rani, Doctor Who). Except in the case of the regeneration into his eighth form (caused by anaesthesia), 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 regeneration, 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 and Susan Foreman suffered from partial amnesia when they arrived in London 1963, apparently caused by challenging the Time Lord rule against interfering when they left Gallifrey. (PROSE: Time and Relative) According to another account, this memory loss occurred when the First Doctor stole the TARDIS and it assaulted his mind. (PROSE: Echoes of Future Past)

• The First Doctor, Ian Chesterton, Barbara Wright and Susan Foreman briefly suffered from amnesia after the TARDIS's fast return switch got stuck, which would have sent it back to the creation of the universe and resulted in its destruction. The ship crashed itself in an attempt to warn its passengers of the dangers, and a small explosion occurred in the TARDIS as the Doctor tried to correct its faulty navigation circuits, rendering all four unconscious. (TV: The Edge of Destruction)

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

• The Masters of Dorada suppressed the memories of the Second Doctor, Ben Jackson, Polly, whom they enslaved when they arrived on Dorada. (PROSE: The Dream Masters)

• During the Third Doctor's exile on Earth, the Time Lords suppressed his knowledge of how the TARDIS worked as part of his punishment. They restored this knowledge when they lifted his exile. (TV: The War Games, The Three Doctors)

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

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

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

• The Time Scaphe crew lost their memories after their ship collided with the TARDIS, but regained them after Ace told them the name of their home world, Gallifrey. The TARDIS took the Doctor's memories to keep them safe from the Process, but later returned them to save itself from destruction. (PROSE: Cat's Cradle: Time's Crucible)

• The Seventh Doctor briefly suffered from amnesia when he was thawed out after being frozen in ice for millions of years. (AUDIO: Frozen Time)

• The Eighth Doctor, Charley Pollard and Gideon Crane suffered from amnesia after the TARDIS crashlanded in Malebolgia. (AUDIO: Minuet in Hell)

• The Eighth Doctor suffered from amnesia after the brain-worm attacked him in the Cube. (AUDIO: Something Inside)

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

• The staff of Torchwood Three often employed a drug called retcon to induce amnesia. (TV: 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. (PROSE: Timewyrm: Apocalypse)

• Tricky Van Baalen lost his memory in a salvaging accident. (TV: Journey to the Centre of the TARDIS)

Instances of functional amnesia

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

• Alistair Gordon Lethbridge-Stewart suffered from amnesia from 1977 to 1983 as a result of meeting his future self. (TV: 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. (PROSE: The Ancestor Cell)

Other instances of amnesia

• The Fourth Doctor suffered from amnesia after being transported to Deepcity. (PROSE: A Device of Death)
• The Eighth Doctor suffered from amnesia after falling into the Master's trap. (PROSE: The Eight Doctors)
• The Eighth Doctor suffered from amnesia again while on Espero. (PROSE: Halflife)
• The Master suffered from amnesia while on Perfugium. (AUDIO: Master)
• Chris Cwej suffered from amnesia while on Ardethe. (PROSE: Deadfall)
• Emily Blandish suffered from amnesia after being hit on the head by a Daemon. (PROSE: Child of Time)
• The Time Lord Azmael provoked selective amnesia to the Earthling twins Romulus and Remus Sylvest by applying a green circle on their wrists. (TV: The Twin Dilemma)