Muggles' Guide to Harry Potter/Magic/Wolfsbane Potion

Overview
Wolfsbane Potion is a potion that will allow a werewolf to retain their intelligence when they transform, thus rendering them less dangerous.

Extended Description
When a werewolf transforms at the full moon, they become basically a wild animal, and will attack anything in the area including humans; of course, the bite of the werewolf is infectious and will result in the bitten person also becoming a werewolf. Wolfsbane Potion is a very recent discovery; if taken regularly once a day in the week prior to the full moon, it results in the werewolf keeping their mind when they transform, thus making them able to lie quietly in private, rather than running mindlessly and striking out at everything.

The potion itself is very difficult to make; apparently Professor Snape is the only one at Hogwarts who is able to brew the potion. It also reportedly has a unpleasant taste, and sugar renders it ineffective. It is unknown whether it contains the plant Wolfsbane (Aconite or Monks-hood).

The potion was evidently invented only a few years before the story starts, by Damocles Belby, the uncle of Marcus Belby. Damocles Belby was apparently taught Potions by Professor Slughorn.

Analysis
The need for this potion, or something like it, becomes obvious as we read through the Shrieking Shack chapters in Harry Potter and the Prisoner of Azkaban. The Muggle understanding of the Werewolf is quite close to the Wizarding understanding of the beast; it is not safe to have a werewolf in human company at the full moon. Thus, when we discover that Professor Lupin is a werewolf, we must also see that there is some way that his monthly transformation can be controlled or corralled. When he was going to school, some ten to fifteen years before, the only way to manage this was to confine him to a place that he could not get out of in wolf shape, and make it distant enough from people that they would not be overly disturbed by the resulting noise. It is uncertain whether this is an option for the more mature Lupin; but the Wolfsbane potion, by allowing him to keep his intelligence through the transformation, also defuses the pure animal rage he would feel at being locked up.

In the story, the Wolfsbane potion in part highlights the ambivalence of Snape's character. When we first see it, Snape is disdainfully presenting a goblet of it to Lupin. Harry is alarmed because the potion is smoking and he believes that Snape would do literally anything, possibly including killing the current holder of the office, to gain the post of instructor of Defence Against the Dark Arts. After Snape reveals that Lupin is a werewolf, Lupin tells Harry that if Snape had really wanted to cause trouble, it would have been extremely simple for him to do so; a single tiny mistake in mixing the potion, and Lupin, transformed, would surely have injured or killed students.

Greater Picture
The fact that the potion must be taken regularly in the week prior to the full moon also proves critical. It transpires that missing even a single dose of the potion renders it useless, so Lupin's having forgotten it once causes him to transform to a mindless beast in the closing scenes of the book. This allows Peter Pettigrew to escape. Pettigrew's escape is necessary for Voldemort's off-scene return from Albania and rise to power later in the series.

We note that Lupin has a continuing presence in the story: he returns as a member of the Order of the Phoenix, having been mentioned as "a member of the old crowd" by Professor Dumbledore at the end of Harry Potter and the Goblet of Fire, and we see him a number of times up until his death in the final battle. It is uncertain exactly how he is weathering the lunar cycle during this time; we're told his mission is to infiltrate the werewolves under Fenrir Greyback, and during that interval one supposes he must be running with the pack and so unable to use the potion. However, in the final book, with Snape unavailable to brew the potion, Lupin and Tonks marry and live together, without a word as to how that is being accomplished.