Not since the Fifth Doctor has he had to choose between his own life and his companions. In the Series 1 finale, The Parting of the Ways, Rose Tyler had absorbed the powers of the time vortex and was at risk of being destroyed with all the new found power flowing through her body. The Doctor chose to save her (by snogging and removing the powers via salivary contact?) and, in doing so, caused every cell in his own body to die, causing the first regeneration of the NuWho series.