8. A Noble Sacrifice
Many regenerations occur after the Doctor has taken fatal damage and needs to rejuvenate his cells with the notable exception being the Second Doctor who was forced a 'change of appearance' by the Time Lords due to his face being too noticeable on Earth. However, there have only been a handful occasions which have forced the Doctor to sacrifice his life in order to save his current companion, and one is iffy at best. The Ninth Doctor reabsorbed the powers of the time vortex from Rose Tyler before depositing it back into the TARDIS' core before it consumed her. Doing this, however, caused every cell in his body to die and therefore caused a regeneration. It wasn't revealed if he knew this would occur and as such, this is the 'iffy' sacrifice that was mentioned above. However... The Fifth Doctor
knowingly risked his life to save his companion, Peri, in The Caves of Androzani. They were both infected with spectrox which unfortunately is fatal to most creatures. Knowing that the cure is bat milk, he and Peri end up finding some and, just before they are able to take it, the Doctor accidentally spills half of it (typical!) leaving just one dose to spare. With no hesitation, he gives Peri the remaining milk, allowing her to get better but causing him to regenerate into Colin Baker in the process. A noble sacrifice indeed.
