When it comes to plot holes, The Vampire Diaries has many inconsistencies and inexplicable story arcs that lack a plausible explanation, even in the fictional universe it takes place in. In fact, the series even ends on one -- Bonnie removing the Cure for Immortality from Elena. As it was stated early on, if an immortal takes it, they turn into a human, but then if someone removes it from them, they die.

It's well established in The Vampire Diaries that the cure is a single dose which when ingested turns the blood of the one who has taken it into a "living" version of the cure. Then, if the cure is removed from them, whether partially or completely, that immortal-turned-human rapidly ages and can't be saved.

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This is seen in the case of the vampire, Katherine Pierce, who is force-fed the cure by Elena when she attacked her at the end of Season 4. While she is in perfect health, she starts exhibiting symptoms of old age after the cure is drained from her by the immortal Silas. Her teeth start falling off, her hair starts going grey and, in the end, she suffers from a major heart attack that kills her. Upon meeting Silas, Amara then drains the cure from him, but he doesn’t live long enough to suffer the symptoms of rapid aging.

However, after Elena takes the 1994 version of the cure and turns human, all the rules regarding the cure are seemingly dropped. Before she is put into a magically induced sleep by Kai Parker, Damon promises to take the cure and turn human, too. As Elena was now the living version of the cure, for Damon’s plan to work, he would have had to remove the cure from Elena. But her possible death and other side-effects after the removal of the cure are never touched on in the series.

But the rules still remain the same for others. Stefan is turned human after Bonnie removed the cure from Elena and injected him with it. When he takes out the cure from himself via a syringe to turn Damon human, it's explicitly stated that he would have rapidly aged and died even if he had not sacrificed himself to kill Katherine. Then, in The Vampire Diaries spin-off, The Originals, Klaus tells Rebekah the location of the cure, but he states that she will have to wait “a handful of decades” to allow Damon to reach a ripe old age so that the removal of the cure won’t end his human life in between.

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Damon and Stefan Salvatore stand side by side in The Vampire Diaries.

There have been many theories explaining why Elena survived the removal of the cure, with some posing the idea that she was in a magical coma. However, there exists a very simple reason why she survived: a recipient of the cure rapidly ages on its removal if they have lived beyond their natural human years. Katherine had been a vampire for more than 500 years and Stefan and Damon were turned sometime in 1864. Thus, the absence of the cure in their case is their years catching up to them quickly and them dying of old age. The same would have happened in the case of Silas, who was more than 2,000 years old.

Elena was only 18 when she was turned in Season 3, so when the cure is removed from her in Season 8, she does quickly age, but in her case, the sudden spike in her age is only 4-5 years -- turning her the age she would have been had she not been turned into a vampire. Elena still had most of her natural human life span ahead of her, and that’s the reason, even without the cure in her system, she lived on.

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