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In chapter 18 of Harry Potter and the Prisoner of Azkaban, Lupin makes it clear that the potion is taken a week in advance (emphasis mine):

"As long as I take it in the week, preceding the full moon, I keep my mind when I transform."

However, several events later on rely on him not having taken it tonight, the night of the full moon. For example, chapter 19 has

"I’ve just been to your office, Lupin. You forgot to take your potion tonight"

and chapter 20 has

"He didn’t take his potion tonight! He’s not safe!"

So what's going on? If Wolfsbane is to be taken a week in advance, why does it matter that Lupin hadn't taken it tonight? He didn't need to take it tonight, the night of the full moon, and nobody needed to check if he did. This should have all been sorted out roughly a week beforehand.

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    In the week, as every day during that week. – Oni Jun 14 at 23:05
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The first quote you cite is a bit vague. At face value it could seem to mean that the potion need only been taken once during the week in order for it to be effective. However, it could be that the potion needs to be taken throughout the week and this particular quote just doesn’t give all the details.

We can perhaps find support for this possibility from an earlier passage. When we actually see Snape deliver the potion on a prior occasion we find the following exchange:

“I made an entire cauldronful,” Snape continued. “If you need more.”

“I should probably take some again tomorrow. Thanks very much, Severus.”

Here we see that Snape clearly prepared more than one dose, although one could argue that this doesn’t show that more than one dose is necessary, for perhaps it would be saved for future months.

Lupin’s response, though, is more indicative. If it was really true that one dose in the week would suffice, there would be no reason for him to take more the next day. On the other hand, the fact that he only says it as a possibility would seem to imply that taking it the next day might not be strictly necessary.

The resolution then may be that it is not a binary choice of effective or ineffective. Presumably, taking a dose of the potion helps somewhat, but taking more doses throughout the week helps more. When Lupin would anyway lock himself in his office for the full moon, perhaps a dose or two would be enough to prevent any negative occurrences. But when he is out on the grounds with several children and hasn’t taken an additional dose that day, it may well be that he would be considered dangerous enough. Hermione, clever as she is, may be correctly wary of him if he has not taken an additional dose that day.

Alternatively, even if it is only required to take the potion once during the week, that doesn’t mean that Lupin necessarily already took it earlier in the week. It could be that he was planning on taking it the one time on the day of the full moon, and thus skipping it that day would mean that he hadn’t taken it at all, and would therefore be fully dangerous.

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    You could continue to speculate: Maybe its effectiveness declines with time, so if you have no reason not to, taking it at the last minute is best. He is also portrayed as generally disorganized, so it wouldn't be that unbelievable for him to forget or run out of time to take it. – Azor Ahai -- he him Jun 15 at 23:25
  • The licensed mobile game "Harry Potter: Hogwarts Mystery" states outright that the potion has to be taken for one week straight. Of course, canonicity of the game is debatable... – LokiRagnarok Jun 16 at 11:49
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The week includes the first full moon day.

To transform safely, Lupin has to take the Wolfsbane Potion every day in the week preceding the full moon. Presumably if he misses a day of taking the Wolfsbane Potion, he may not be able to keep his mind when he transforms.

“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 am able to curl up in my office, a harmless wolf, and wait for the moon to wane again.”
- Harry Potter and the Prisoner of Azkaban, Chapter 18 (Moony, Wormtail, Padfoot and Prongs)

Lupin missed one day of taking the Wolfsbane Potion in the week preceding the full moon, which happened to be the night of the full moon, so the potion did not have the intended effect.

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    This may be the correct answer, but the quote doesn’t actually say that it must be taken every day. – Alex Jun 15 at 0:21
  • @Alex: The quote doesn't explicitly specify a daily frequency, but which frequency the potion is to be taken at doesn't really change the gist of the answer, i.e. that the potion needs to be taken over the course of the week, at the intended interval (which we don't explicitly know), as opposed to OP's interpretation of taking it once a week in advance. – Flater Jun 15 at 9:09
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    If I say "I have to go shopping in the next week", that would mean "I have to go shopping at least once in the next 7 days". So that quote doesn't seem to imply "every day". – NotThatGuy Jun 15 at 11:13
  • @Flater The quote doesn’t explicitly specify that there’s a frequency at all. As I said, it might be true (and indeed I argued something similar in my own answer), but this quote doesn’t demonstrate that. In fact the question referenced this very quote. – Alex Jun 15 at 11:14
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    @NotThatGuy On the other hand "As long as I take my birth control pills in the week before having sex" would imply having to take them every day. So I'm not sure proposing sentences with different semantics helps disambiguate the one in question. I think it's sufficiently ambiguous. (Caveat: my dialect doesn't use "in the week before..." also I don't know of any pills that are effective in a week, so don't take it as medical advice). – Azor Ahai -- he him Jun 15 at 17:15
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You're assuming that the effect (i.e. the protection) activates the same way as it deactivates, but that is not necessarily the case. To prove that point, I'll use a real-world example: ketosis. Coincidentally, it uses the same week-long period as the wolfsbane.

A keto diet consists of avoiding carbs at all costs (there's a very low daily limit). If you stay under that limit for about a week, your body becomes carb-starved and starts burning fat. That state of your metabolism is called ketosis, and being in that state is the goal of the diet, so you burn fat more rapidly.

However, if you go over the daily carb limit just one day, the ketosis stops immediately. To go back into ketosis, you need to say under that daily carb limit for about a week again.


Based on all quotes and evidence provided in both the question, answers and comments, I see no reason to exclude the possibility that the wolfsbane effect behaves the same way as ketosis does, i.e. that it requires to be taken for several consecutive days for the effect to activate, but failing to take it for a single day deactivates the effect.

"As long as I take it in the week, preceding the full moon, I keep my mind when I transform."

You're reading this as "I must take it once, a week in advance", but it can also mean "I must take it [daily] for a week".

Note that the frequency (e.g. daily, hourly, ...) isn't specified. I'm assuming it is to be taken on a daily frequency for the sake of keeping things simple, but the answer remains the same regardless of what the exact frequency is.

"He didn’t take his potion tonight! He’s not safe!"

This seems to confirm that not taking the potion immediately (at least within some hours) deactivates the effect.

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I get really bad allergies. I have to start taking my allergy pill about 2 weeks before allergy season starts for it to be maximally effective.

If I skip a day in the middle of allergy season. I might be better off than if I never took an allergy pill but you can bet I am still going to be miserable.

I assume Wolfsbane works a similar way.

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