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In the second book/movie, Hermione used a cat's hair in her Polyjuice Potion by mistake and her body got covered in fluffy hair and she possibly even got a tail.

But, J. K. Rowling writes on Pottermore:

You can change age, sex and race by taking the Polyjuice Potion, but not species.

Where's the catch? Yeah, Hermione didn't turn fully into a cat, but she didn't remain a human either. Has anyone ever asked JKR about this?

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    I take this as meaning that Polyjuice potion should not be used to change species. It's not what it's designed for, and it doesn't work reliably. If it's for sale, "do not use this to try to change species" is probably on a yellow sticker on the bottle.
    – DaaaahWhoosh
    Commented Nov 6, 2017 at 17:15
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    @DaaaahWhoosh "If it's for sale", then the Ministry of Magic would have filters against Polyjuice Potion in their office access.
    – Arturo Torres Sánchez
    Commented Nov 6, 2017 at 23:23
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    Hair + tail: she just turned into a man.
    – Crouching Kitten
    Commented Nov 8, 2017 at 0:34

3 Answers 3

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Hermione was still human - just with catlike features and fur.

Hermione never actually turned into a cat, changing species - she simply mutated, becoming more catlike in appearance.

“Hermione let her robes fall and Ron backed into the sink. Her face was covered in black fur. Her eyes had gone yellow and there were long pointed ears poking through her hair.

‘It was a c-cat hair!’ she howled. ‘M-Millicent Bulstrode m-must have a cat! And the P-Potion isn’t supposed to be used for animal transformations!”
- Harry Potter and the Chamber of Secrets, Chapter 12 (The Polyjuice Potion)

In addition, when used with cat hair, it didn’t wear off like it was meant to.

Polyjuice Potion is supposed to wear off on its own after an hour. However, Hermione’s Polyjuice cat transformation took weeks to cure, even under the care of Madam Pomfrey.

“Hermione remained in the hospital wing for several weeks. There was a flurry of rumour about her disappearance when the rest of the school arrived back from their Christmas holidays, because of course everyone thought that she had been attacked. So many students filed past the hospital wing trying to catch a glimpse of her that Madam Pomfrey took out her curtains again and placed them around Hermione’s bed, to spare her the shame of being seen with a furry face.”
- Harry Potter and the Chamber of Secrets, Chapter 13 (The Very Secret Diary)

It’s very clearly not a successful transformation into a cat, and in fact, this sort of result seems to be exactly why Polyjuice Potion is said not to work to change species - because it doesn’t fully transform a human into an animal, and it requires medical care to change fully back.

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If we read it as "You cannot change into a different species successfully," there is no contradiction.

We can be fairly certain that she did not forget Hermione turning into half cat since it was a major plot point.

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    I read it with an implied safely, but yours also works.
    – Segfault
    Commented Nov 8, 2017 at 0:34
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No, JKR did not contradict herself on the polyjuice potion with regards to the effect of Hermione's potion.

According to Hermione, the polyjuice potion would require a bit of people

"We still need a bit of the people [emphasis mine] you're changing into..."

-Harry Potter and the Chamber Of Secrets, Ch12, Pg213

Merriam Webster makes it pretty clear that a person (singular form of people) must be a human. Ergo, since Hermione's potion did not contain a part of a human, it was not in fact a polyjuice potion. Since Hermione's potion was not a polyjuice potion, any resultant effects of it could not be applied to the traditional understanding of a polyjuice potion.

What would be an interesting academic question is what would happen if a person were to use a portion of a transformed animagus -- What would have happened if for instance Ron's potion had Scabber's/Pettigrew's hair?

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  • @Sidney 'What would be an interesting academic question' I believe it is either 'strictly academic' or 'purely academic' but I might be remembering the wording wrong. Either way Horace Slughorn would probably agree with you esp as he was potions master (though perhaps he or Severus would know; certainly possible). I find it an interesting thought too but although I could take guesses on how to answer the riddle it'd still be speculation. ... and in any event that doesn't contribute to the answer does it. Tend to forget that.
    – Pryftan
    Commented Nov 7, 2017 at 0:51
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    Defining terms to avoid answering the question doesn't make for a great answer.
    – Matthew Read
    Commented Nov 7, 2017 at 0:56
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    This is just splitting hairs. (...Is that pun? Should I say something about whether it's intended or not?)
    – jpmc26
    Commented Nov 7, 2017 at 1:50
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    @Pryftan, the difference between "strictly academic" and "purely academic" is strictly puristically academic.
    – Wildcard
    Commented Nov 8, 2017 at 2:45
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    @Sidney I agree with Matthew, giving it a moment's thought. You have to remember that in a magical world person might mean something else entirely; and what about humanoids? Do they not count as a person? Maybe in the mind of some but in my mind they are certainly considered a person. And they're also a living creature, a being. But even then the potion was still Polyjuice; putting the wrong type of hair in doesn't change the fact it's still the same potion. The hair is just a way of saying for this invocation of the spell (or if you prefer quaff of the potion) change me into ...
    – Pryftan
    Commented Nov 9, 2017 at 18:12

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