In Prisoner of Azkaban, when Harry follows Sirius into the tree, there’s a conversation involving Snape, Sirius and Lupin that reveals Peter Pettigrew was still alive.

When the cloud moves, there was a full moon and Lupin becomes a werewolf. Why didn’t he take his Wolfsbane Potion?

  • 3
    I don't have the books to hand, so I can't give you the direct quote, but Snape implies that he brings Lupin the Wolfsbane Potion. That's how Snape gets involved that night, he arrives at Lupin's office, but Lupin is missing, sees him on the Marauders Map, assumes the worst and goes to help.
    – CyanAngel
    Commented Jun 9, 2014 at 12:02

2 Answers 2


When Snape turns up in the Shrieking Shack, he has this to say to the assembled party:

Snape was slightly breathless, but his face was full of suppressed triumph. “You’re wondering, perhaps, how I knew you were here?” he said, his eyes glittering. “I’ve just been to your office, Lupin. You forgot to take your potion tonight, so I took a gobletful along. And very lucky I did... lucky for me, I mean. Lying on your desk was a certain map. One glance at it told me all I needed to know. I saw you running along this passageway and out of sight.”

It sounds like Snape found the goblet sitting on his desk, about to be drunk, and Lupin was also inspecting the map at the same time. (If the map was inactive, it’s unlikely Snape would have been able to activate it.)

Later, Lupin explains the Wolfsbane Potion to Hermione:

“The potion that Professor Snape has been making for me is a very recent discovery. It makes me safe, you see. As long as I take it in the week, preceding the full moon, I keep my mind when I transform… I’m able to curl up in my office, a harmless wolf, and wait for the moon to wane again.”

In the excitement and uproar of seeing Sirius and Peter on the map (especially given that he thought Peter was dead), he probably left the potion (and the map) as he want to assist Sirius and/or Harry, Ron and Hermione.

  • 2
    This does lend itself to the question then of if he had the entire previous week to take the potion then why was it still sitting undrunk on his desk at that point? Pure forgetfulness seems to be the answer as far as I can see...
    – Chris
    Commented Jun 9, 2014 at 14:39
  • 7
    @Chris perhaps it must be taken nightly for a week straight.
    – nnnn
    Commented Jun 9, 2014 at 15:46
  • 3
    @nnnn: Ah yes. I suppose you could read it as "As long as I take it (continuously) in the week". I'd read it as "As long as I take it (once) in the week".
    – Chris
    Commented Jun 9, 2014 at 15:48

He didn't forget about the full moon he merely forgot to take his potion. He has to take the potion each night for the whole cycle of the full moon.

  • I'm trying to bolster up the answer with some quotes but I don't have my books and google is not proving to be my ally at the moment. Commented Jun 9, 2014 at 12:05
  • 6
    Agreed, not having taken his potion is actually the reason Professor Snape came after him: he's brought his potion. Incidentally, in Goblet chapter 35, Professor Dumbledore predicts that Barty must have forgotten to take his potion “in the excitement of tonight”, so apparently this is a common vulnerability of wizards.
    – b_jonas
    Commented Jun 9, 2014 at 12:33
  • 3
    @b_jonas It's a vulnerability of everyone no matter how routine a task is for people they will still forget occasionally. Taking a tablet or a pill for instance. Commented Jun 9, 2014 at 13:12

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