In Harry Potter and the Prisoner of Azkaban, Harry and Lupin could practice the patronus on the boggart, but when Harry was in the final task in the Harry Potter and the Goblet of Fire, he tried to use the patronus on the boggart, it didn't work and he then moved on to use ridikkilus.

Why could he use a patronus against the boggart, but then not in another?


2 Answers 2


In Harry Potter and the Prisoner of Azkaban, his patronus was working on the Boggart (blocking it from approaching him) it just wasn't very effective in pushing it away from him:

To make matters even worse, Harry’s Anti-Dementor lessons were not going nearly as well as he had hoped. Several sessions on, he was able to produce an indistinct, silvery shadow every time the Boggart-Dementor approached him, but his Patronus was too feeble to drive the Dementor away. All it did was hover, like a semi-transparent cloud, draining Harry of energy as he fought to keep it there. Harry felt angry with himself, guilty about his secret desire to hear his parents’ voices again.
Harry Potter and the Prisoner of Azkaban

By the time we see him use another Patronus on a Boggart, it's become fully corporeal and capable of interacting with solid matter. Instead of hovering like a vague silver cloud it's now quite literally erupting from his wand in the form of a charging stag. Clearly it's a vastly more powerful spell:

A silver stag erupted from the end of Harry’s wand and galloped towards the Dementor, which fell back, and tripped over the hem of its robes … Harry had never seen a Dementor stumble. ‘Hang on!’ he shouted, advancing in the wake of his silver Patronus, ‘you’re a Boggart! Riddikulus!’ There was a loud crack, and the shape-shifter exploded in a wisp of smoke. The silver stag faded from sight. Harry wished it could have stayed, he could have used some company … but he moved on as quickly and quietly as possible, listening hard, his wand held high once more.
Harry Potter and the Goblet of Fire

  • 6
    So, if I understand correctly, Patronuses are effective at pushing away Dementor-Boggarts, but Riddikulus makes them disappear? Which makes sense - Patronuses don't even make Dementors disappear, as they can't die. So he used the Patronus to push the Boggart away, but once he realized it was a Boggart, there was a more effective spell he could use.
    – trysis
    Dec 27, 2014 at 15:19
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    I suspect a Patronus is effective as a physical barrier to many/most things, not just boggarts or dementors.
    – TylerH
    Dec 27, 2014 at 19:12
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    @TylerH - A corporeal patronus is literally able to interact with solid matter. Imagine being hit by a charging stag.
    – Valorum
    Dec 27, 2014 at 19:21
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    @Richard While I agree, I don't think that's ever mentioned in the books. The references to corporeal patronuses seem to be referring to "fully-formed" (ie having a body vs being a shield or just a wisp of smoke) patronuses rather than "tangible".
    – TylerH
    Dec 27, 2014 at 22:18
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    @TylerH - From 'Fantastic Beasts'; "I knew that I was about to lose consciousness completely as I suffocated. Desperately. I mustered up my last reserve of energy. Pointing my wand away from myself into the deadly folds of the creature, summoning the memory of the day I had been voted President of the local Gobstones Club. I performed the Patronus Charm. Almost at once I felt fresh air upon my face. I looked up to see that deathly shadow being thrown into the air upon the horns of my Patronus. It flew across the room and slithered swiftly out of sight."
    – Valorum
    Dec 28, 2014 at 2:02

To directly answer. It did work the second time, but as someone pointed out, he chose to use the other, more effective spell.

  • Can you provide a source that the patronus charm worked on the boggart in the Goblet of Fire?
    – Edlothiad
    Nov 10, 2017 at 10:11

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