With Doctor Who are excited to see what the program has in store. It's understood that David Tennant's Fourteenth Doctor will only be around for a short period of time, but fans plan on treasuring his take as long as they can.
While Regeneration is often viewed from the lens of creative change and as a summation of what an actor's experience was like on Doctor Who, it's also a painful process. Fans are forced to say goodbye to their favorite character, while the Doctor themselves is sometimes subjected to unimaginable physical and mental torment right before they change faces.
14 War Doctor
Despite his role in the Time War, the War Doctor was not treated to a violent death but a peaceful ing. In Doctor Who's original timeline, he was responsible for the destruction of Gallifrey, yet following the events of "Day of the Doctor," he was successful in saving the planet before his body decided to regenerate following years of use.
While the War Doctor was far from the only incarnation of the character to regenerate due to age, his ing was welcomed by the character. His body decided to heal itself despite a lack of (notable) physical deterioration, and he left the program at peace with himself following years of guilt from the Time War.
13 First Doctor
Like the War Doctor, the First Doctor's regeneration was prompted by his body wearing a bit thin, though unlike his successor, he had doubts about changing. The character was notably frightened of changing, and fell ill during his final adventure, "The Tenth Planet."
Despite being unconscious for an extended period of time, the First Doctor was able to regain some of his strength to save the day before going off into the TARDIS. His ing due to old age was relatively peaceful, though his exposure to Antarctica's extreme cold and collapsing on the floor likely led to some discomfort during his final moments.
12 Eleventh Doctor
The most youthful and eccentric incarnation of the Doctor was ironically the one who aged the most during his tenure. He started out as a 907-year-old Time Lord and ended his tenure around 2,000 years old, having spent the last 900 years of his life defending Trenzalore.
Given how long the Eleventh Doctor was in his body, it's no surprise that he regenerated due to old age, yet he was forced to endure his body longer than other incarnations due to a lack of regenerations. Only when the Time Lords granted him a new set of lives was he finally able to restore his body and change into his next incarnation.
11 Second Doctor
In many respects, it's hard to gauge how painful the Second Doctor's death was because it is one of only two that occurred off-screen. He was forced by the Time Lords to change his face as punishment for meddling in the affairs of other planets, yet was never shown to regenerate.
Despite the uncertainty in what the Second Doctor went through right before his regeneration, dialogue in "The Power of the Doctor" hinted at the procedure of forced regeneration as being very horrifying and akin to lethal injection.
10 Thirteenth Doctor
Jodie Whitaker may have only just departed from Doctor Who, but her regeneration episode will be ed on Twitter for an extensive period of time. Her incarnation's body was infamously hijacked by the Master for much of the story's runtime, but the actual cause for her regeneration came when she was blasted by a Qurunx energy beam.
While getting shot by a giant laser is fairly painful, the Thirteenth Doctor was able to stave off regeneration for some time so that she could spend time with Yaz. It made for a sweet moment, though undermined the severity of the Thirteenth Doctor's injury.
9 Seventh Doctor
Though the Seventh Doctor's death is far from the most painful, it is the most horrifying. The conniving trickster was shot after his TARDIS landed in the middle of a shootout, and the Time Lord was rushed to a hospital where Dr. Grace Holloway killed him on the operating table when she accidentally clogged a vein.
While the Seventh Doctor was under anesthetic for much of the operation, he was awoken by the medical team after he flat-lined, suggesting he felt everything that Grace did to him. Worse still, his regeneration was delayed as a result of the anesthetic, meaning the Doctor actually stayed dead for an extended period of time.
8 Sixth Doctor
One of several incarnations to be done in by radiation, the fate of the Sixth Doctor was left ambiguous for the longest time. Colin Baker refused to film a regeneration scene after being fired from the role, and his take on the character was shown to regenerate without any clear reason at the beginning of "Time and the Rani."
Despite theories that the Sixth Doctor had died falling off of his exercise bike, expanded media gave the character a noble sendoff where he deliberately exposed himself to radiation so the Valeyard would not be able to possess his body. It was physically painful, but the Doctor could at least be comforted by the fact that he sacrificed himself to save the galaxy.
7 Twelfth Doctor
Despite being one of the least likable incarnations of the Doctor, Peter Capaldi's incarnation of the character was arguably one of the most sensitive, and this is reflected in his final episodes. He initially refuses to regenerate after being electrocuted by a Cyberman because he wants to save Bill and the people of Floor 0507.
Beyond the slow excruciation of his body dying from electrocution, the Twelfth Doctor's regeneration was hastened by shots he took from invading Cybermen and by blowing himself up. Even after all this, he still chose to hold on to life, despite how excruciating it was to move around in his war-torn body.
6 Tenth Doctor
For many audiences, the Tenth Doctor's regeneration remains the most emotionally painful for them. The same goes for what the Tenth Doctor went through, as he managed to survive a confrontation with the Master and Rassilon, only to have his life cut short by sacrificing his life to save Wilfred Mott.
While the emotional head space of the Tenth Doctor was raw given he was giving up his last regeneration to save a silly old man, the physical torment was also fairly severe. The Tenth Doctor exposed himself to a powerful burst of radiation and lived with it for an unspecified period of time (likely a few days) as he visited his former friends before finally regenerating.
5 Fifth Doctor
The Fifth Doctor, despite being one of the blandest incarnations of the character, had arguably the best Doctor Who regeneration story. He and his recent companion, Peri, landed on the planet Androzani Minor during the middle of a drug and were exposed to Spectrox poisoning.
Though the effects of the poisoning were minimal at first - rash and cramps - it eventually escalated into paralysis of the thoracic spinal nerve, hallucinations, and a raging fever. The Fifth Doctor suffered through all this while getting shot at and avoiding mud bursts to save Peri and nearly died from burning up despite his ability to regenerate.