Before Harry, Hermione, and Ron drink the Polyjuice potion during the Chamber of Secrets, Hermione states that the effects will only last an hour.

From an early draft of the script (this scene is in the movie):

Harry and Ron nod, glance at the cauldron. The potion resembles a thick, dark, bubbling mud.

HERMIONE: I'm sure I've done everything right. It looks like the book said it should. Once we've drunk it, we'll have exactly one hour before we change back into ourselves.

From the same script (the hospital part of this is now a deleted scene):

Even in shadow, they can see: Hermione's face is covered in FUR, her eyes YELLOW, and POINTED EARS poke through her hair.

HERMIONE: It was cat hair I plucked off Millicent Bulstrode's robes! Look at my face!

RON: Look at your tail.



Just then, Hermione's tail twitches INTO VIEW.

RON: Is that thing ever going away?

HERMIONE: Any day now, according to Madam Pomfrey. I'm just thankful I've stopped coughing up fur balls.

In this answer, @JasonBaker theorizes that it's the dosage size that might affect the duration, but all three drank roughly the same amount before dropping their glasses and transforming. And in the final cut of the film, Ron still mentions that Hermione will be in the infirmary for several days waiting for the potion to wear off.

Even Pottermore says it shouldn't last that long:

The effect of the potion is only temporary, and depending on how well it has been brewed, may last anything from between ten minutes and twelve hours.

So what went wrong with Hermione's portion of the potion? Was it simply because she accidentally used cat hair instead?

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    Yeah, in the book it was all down to just the fact that it wasn't a human ingredient. It was never intended to work with an animal hair, so, messy side effects, as shown.
    – Radhil
    Commented Jul 7, 2017 at 17:23
  • 3
    The Pottermore writings from JKR tend to go with book canon. Don't expect them to match the movies.
    – ibid
    Commented Jul 7, 2017 at 20:03

3 Answers 3


It was simply the use of cat hair instead of human hair.

As Professor Snape says to his class, potion-making is a subtle science and exact art. Changing even a little thing can cause the effects of a potion to differ drastically from the intended or usual effect. Polyjuice Potion is already a difficult to brew potion, and presumably brewing it is as much as if not more of an exact science as brewing more simple potions. Adding hair from a cat instead of hair from a human caused the potion to backfire and have unintended effects, because the Potion wasn't designed to be used to transform into an animal.

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)

The hair is what determines the form that someone will take when using Polyjuice Potion. It's one of the most important (arguably the most important ingredient) in the potion.

In Harry's first Potions class with Snape, we see such an example of a potion going terribly wrong because of a seemingly minor change.

“Neville had somehow managed to melt Seamus’s cauldron into a twisted blob and their potion was seeping across the stone floor, burning holes in people’s shoes. Within seconds, the whole class were standing on their stools while Neville, who had been drenched in the potion when the cauldron collapsed, moaned in pain as angry red boils sprang up all over his arms and legs.

‘Idiot boy!’ snarled Snape, clearing the spilled potion away with one wave of his wand. ‘I suppose you added the porcupine quills before taking the cauldron off the fire?’ Neville whimpered as boils started to pop up all over his nose. ‘Take him up to the hospital wing,’ Snape spat at Seamus.”
- Harry Potter and the Philosopher's Stone, Chapter 8 (The Potions Master)

The potion was one that was supposed to cure boils, but Neville Longbottom adding the porcupine quills before taking the cauldron off the fire caused it to melt a cauldron, burn through shoes, and make Neville break out in boils. A simple potion to cure boils, and keep in mind a potion considered safe enough to let first-years brew on their first day of Potions class, is certainly not supposed to have such explosive effects if brewed correctly.

The Dark Lord has some more relevant quotes in his answer that show the aftereffects of Hermione taking the Polyjuice Potion and how her return to human form progressed in the care of Madam Pomfrey.

The effects of the Polyjuice Potion with cat hair were already quite different from the intended effect, even without considering the duration it lasted. Hermione didn't turn into Millicent Bulstrode, or another human, which is obviously because the transformation is directly related to the hair used and no human hair was used in Hermione's dose of Polyjuice Potion to make that possible. However, she also didn't properly transform into a cat the way Professor McGonagall would when using her Animagus form. Instead, she turned into something with features of both a cat and a human, an odd mix between the two.

Still of Hermione half-transformed into a cat from the movie

In addition, what also shows that it's only the use of cat hair that caused the potion to last so long is that Harry and Ron took the exact same potion at the exact same time as Hermione did. The only variable in their separate doses of potion were the hairs. Harry had Goyle's, Ron had Crabbe's, and Hermione had a cat's. When Ron and Harry took theirs, it worked exactly as intended, with no unusual effects, and it lasted for the usual amount of time. Hermione's dose of potion, however, turned her into a half-cat and lasted much longer than Polyjuice Potion is supposed to last.

  • 1
    The question isn't so much concerned with the form she took, but the duration it lasted. Is the fact that it was a cat hair the sole error causing it to last days instead of the expected one hour?
    – phantom42
    Commented Jul 7, 2017 at 17:25
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    @phantom42 Yes, I'm fairly sure it is, since the potion wasn't designed to be used with animal hair, and using it in that way could cause unusual side effects. Potions are a complicated science, and changing even a small part of the mixture can cause it to have drastically different effects.
    – Obsidia
    Commented Jul 7, 2017 at 17:33
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    @phantom42 I've added quotes and more information than supports the conclusion, and makes it a bit clearer how a mistake in a potion could cause unintended effects that may be quite different from the potion's usual effects.
    – Obsidia
    Commented Jul 7, 2017 at 18:54
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    I agree with this answer. I think it's quite clear from the text that it's just the cat hair. The potion isn't meant to be used for animal transformations, so the potion had unintended side effects instead of just turning Hermione into a cat rather than Millicent. Hermione is quite adept at most magic, so it's not like her to get the brewing wrong.
    – tobiasvl
    Commented Jul 7, 2017 at 19:06
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    @tobiasvl Thanks! :) I thought it was quite clear from the text that it's the cat hair causing it. That's drastically different from the potion's intended use. The way Hermione definitively says it's not supposed to be used for animal transformations suggests that the instructions may have even specifically warned against attempting to use it that way. In addition, Harry and Ron both took the potion from the same batch at the same time she did and nothing went wrong for them. The only way Hermione's dose of potion differed from theirs was the use of cat hair.
    – Obsidia
    Commented Jul 7, 2017 at 19:16

The use of cat hairs meant that she didn't transform back after one hour.

Others have made this point but I thought I'd add an answer with the relevant quotations from the books.

There was nothing wrong with Hermione's Polyjuice Potion. Harry and Ron transformed successfully and returned to their original forms after one hour as planned. Hermione got stuck with the appearance of a cat because the Potion is not designed to be used with animal ingredients.

"It was a c-cat hair," she bowled. "M-Millicent Bulstrode m-must have a cat! And the P-Potion isn't supposed to be used for animal transformations!"
(Chamber of Secrets, Chapter 12, The Polyjuice Potion).

This meant that Hermione was out of action for a while.

Hermione remained in the hospital wing for several weeks.
(Chamber of Secrets, Chapter 13, The Very Secret Diary).

Madame Pomfrey was able to heal her, but only gradually over a long period of time.

[Hermione's] spirits were greatly improved by the fact that all the hair had gone from her face and her eyes were turning slowly back to brown.
(Chamber of Secrets, Chapter 13, The Very Secret Diary).

Hence Hermione had to stay in the hospital wing until the misstep with the cat's hair had been put right by Madame Pomfrey.

The film also has Hermione staying in the hospital wing for an extended period, although the scene in question didn't make it into the final movie. It's below at 13:17.


The potion is complicated and can go awry. For example, when Hermione brewed Polyjuice in her second year, she intended to take the form of Millicent Bulstrode, but she mistook a cat's hair for Millicent's. The resulting potion caused Hermione to partially transform into a cat. As the Polyjuice Potion cannot be used to successfully transform into an animal, Hermione's change did not reverse after an hour. She went to the Hospital Wing and did not recover until after the winter holidays.[2] Found this on


I found two more posts where the answers state it can only be used to transform into another person however they do not say that it would extend the duration of the potion.

How do Animagi and Polyjuice Potion mix in Harry Potter?

Is Polyjuice Potion a Form of Transfiguration?

  • 4
    So you have any resource besides the wikia? I hear that's not very reliable. Commented Jul 7, 2017 at 17:31
  • Added the other info I found but can't seem to find anything that states animal hair directly caused the duration other than that it can cause side effects.
    – Scath
    Commented Jul 7, 2017 at 17:56

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