T'Challa must fight any challengers to be accepted as King of Wakanda after his father dies. After that, he becomes the Black Panther.

Several scenes in the 2018 movie, Black Panther, show he already has the super-strength and super-speed of the Black Panther before the challenge.

Why was he allowed to accept the plant juice that transforms his body before he became king?

I can accept answers from other Black Panther material besides the movie as long as the source is canon.

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    I am 99% sure T'Challa was already the Black Panther when his father was King; likely due to old age(?)... I don't think that is covered. But T'Challa would have become the interim Black Panther after his father's death to continue the presence of the protector, similar to how T'Challa was the interim King until such a time as he could be officially crowned. Just because the King has died does not mean there can not be a line of succession until he is replaced. The best, related, example I can think of off the top of my head is the Green Lantern ring searching for a successor after a death. – Odin1806 Mar 10 '18 at 21:27

As explained in this answer, you are mixing together two separate events. It's a common mistake, the movies aren't entirely clear about it, but they do make a distinction between:

  • Being the Black Panther
  • Being the King of Wakanda.

If you pay attention to T'Challa's dialogue to Natasha in Civil War, you'll hear him say something like "I have been a warrior for my people, and now with my father's death I am also king."

The title/mantle of Black Panther is not automatically held by the King of Wakanda. They are frequently the same person, and being King apparently entitles one to become the Black Panther, but in the prequel comics (linked in my other answer), we see that T'Chaka voluntarily steps down and passes that title on to T'Challa nearly a decade before his death, roughly at the same time that Tony Stark is revealing his identity in Iron Man. T'Challa had been the Black Panther for a long while by the time he became King, which is why they have a specific step in the ritual to take away that power.

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