Muggles' Guide to Harry Potter/Magic/Polyjuice Potion
|Muggles' Guide to Harry Potter - Magic|
|Features||differs based on subject|
|First Appearance||Harry Potter and the Chamber of Secrets|
Polyjuice Potion allows the taker to assume the physical appearance of another person. The potion takes a month to brew, and must include part of the person you wish to look like.
Extended Description 
One of the ingredients of Polyjuice Potion is boomslang skin. The boomslang is a very real venomous snake, native to Africa.
The potion is used in Harry Potter and the Chamber of Secrets by Harry and Ron to question Draco about the Heir of Slytherin, with almost no result. Hermione, by accidentally attempting to change into a cat, demonstrates the problems with using potions without being quite sure of the effects they will have.
Finally, it is used extensively by Harry, Ron, and Hermione in Harry Potter and the Deathly Hallows when they need to disguise themselves. It is used first by Mad-Eye Moody to create six replicas of Harry as a way of confusing the Death Eaters who are trying to stop his departure from the Dursleys'. It is then used by Harry to disguise himself as "Cousin Barny" at the wedding of Bill and Fleur. Harry, Ron, and Hermione use it to enter the Ministry, Harry and Hermione use it to visit Godric's Hollow incognito, and Hermione uses it to enter Gringotts disguised as Bellatrix Lestrange.
It must be extremely useful to appear to be someone else. The potion is remarkably comprehensive in its actions; the false Moody, transformed by the potion, is missing an eye and a leg, and these grow back when he is deprived of the potion. In fact, one assumes they must grow back each night, as the effects of the potion last only an hour. One must question whether the suppressed limb must be exercised to avoid it atrophying... did the false Moody have to exercise each day in his normal shape to avoid the effects of nine months without moving the suppressed leg?
The process of changing to a new shape by means of the potion is described as being quite painful, while the reversion to original self is so painless that neither Ron nor Harry notices it happening in themselves. As the false Moody was not apparently in pain when he drank from his hip flask (which contained, apparently, the Polyjuice Potion that retained his appearance), we can assume that it is the process of changing that is painful; maintenance in the unnatural shape should be much less so.
Greater Picture 
One wonders what happened when Crabbe or Goyle, standing guard duty, ran out of time? They did not seem to have any additional potion with them, and it is certain that a 6th-year Crabbe or Goyle would not have fit into the robes of the students they chose to disguise themselves as, which were typically first- to third-year girls. Possibly they had additional stock of prepared potion with them, but we never see them drinking any of it. However, throughout the seventh book, the effects of the potion seem to wear off at much more convenient times than we would expect; rather than a fixed hour in duration, for instance, Harry's disguise as "Barny" seems to last for the duration of the wedding, and Harry, Ron, and Hermione's disguises as they invade the Ministry not only seem to remain in place for the entire time that they are in the Ministry, but seem to wear off very conveniently just as they leave Grimmauld Place for the forest where the Quidditch World Cup had been played two years before. This convenient duration of the potion effects seems to be one of a number of places where the author is altering the magic she has created in order to make a better story.