The regenerative cycle creates a large amount of energy that engulfs the Time Lord's body. The Doctor has shown the extent of this energy when being able to rapidly regrow a severed hand in the first fifteen hours of his regeneration.
The Doctor frequently experiences periods of instability and partial amnesia following regeneration.
The TARDIS appears to aid in the regenerative process. Of the occasions the Doctor regenerates outside the TARDIS, one was forced on him by the Time Lords (The War Games), one required a Time Lord to give the Doctor's cells a "little push" to start the process (Planet of the Spiders), and one needed the TARDIS Zero Room to help him recover (Castrovalva)
Here is how each Doctor's regeneration came about:
- First Doctor: Apparently succumbed to old age, steadily growing weaker throughout 'The Tenth Planet' and collapsing at the end of the episode.
- Second Doctor: Was exiled to Earth by the Time Lords at the end of 'The War Games'. He was forced upon a "change in appearance" along side the exile.
- Third Doctor: Gained radiation poisoning from the Great One's cave of crystals at the end of 'Planet of the Spiders.'
- Fourth Doctor: Fell from the Pharos Project radio telescope in 'Logopolis.'
- Fifth Doctor: Caught a disease known as spectrox toxaemia, from an acidic substance. He gave up the antidote to his current assistant, sacrificing himself in 'The Caves as Androzani.'
- Sixth Doctor: When the Rani attacked the TARDIS and caused it to crash land, he suffered unspecified injuries. This was at the start of 'Time and the Rani.'
- Seventh Doctor: At the opening of the '1996 movie', he was shot. Although the gunshot wounds were non-life threatening, he was killed during surgery as the doctors who had to operate on him were unfamiliar with Time Lord physiology.
- Eighth Doctor: Not revealed as yet (It can be assumed thatthe doctor died after being inflicted with serious injuries during the Time War.)
- Ninth Doctor: In the 'Parting of the Ways', the Doctor absorbed the energy of the TARDIS' time vortex in order to save Rose. Because of this, he began to experience cellular degeneration and had to regenerate.
- Tenth Doctor: Current incarnation.