Polyjuice Potion

Polyjuice Potion is a complex potion which transforms a person to look exactly like someone else. One dose lasts for one hour, but doses can be renewed by continuing to drink the potion.

The instructions for making this potion, which takes over a month to complete, are found in the book Moste Potente Potions (CS9).

Ingredients

• lacewing flies stewed 21 days
• leeches
• powdered bicorn horn
• knotgrass
• fluxweed picked at full moon
• shredded boomslang skin
• a bit of who one wants to turn into: toenail clippings, dandruff, etc. (CS10, CS11, Pm)

Preparation

• Takes a month to brew because the lacewing flies have to be stewed for 21 days, and fluxweed can only be picked during the full moon (CS9).
• Before the last ingredient is added - the bit of the person being transformed into - the potion looks like thick dark mud and bubbles sluggishly (CS12, HBP9).
• When the last ingredient is added, the potion froths a great deal and changes colour, but the resulting colour varies depending on the person supplying the last ingredient (CS12).

References from the canon

• Isn't supposed to be used for transforming humans into animals (CS12); if misused for that purpose, results in an incomplete transformation requiring skilled medical treatment to undo (CS13).
• Polyjuice has a "slight overpowering fishy residue" and the transformation is painful (CC1.16).
• Newt Scamander took polyjuice potion to transform into his brother Theseus. "Just enough to get me inside" the French Ministry of Magic (CG87).
• Polyjuice Potion can be used for cross-gender transformations. Crouch Jr. used Polyjuice Potion to impersonate his mother in order to escape from Azkaban, while she used it to impersonate him (GF35). Crabbe and Goyle appear to have used it periodically during their sixth year to pass themselves off as much younger, smaller girls in order to be less conspicuous (HBP21).
• Crouch Jr. used Polyjuice Potion to impersonate Moody, so it can account for missing limbs and scars as well as age differences (GF35).
• Crouch Jr.  had broken into Snape's office to steal Polyjuice ingredients, but Snape accused Harry, who had stolen the same ingredients in their second year along with Hermione and Ron. (GF27).
• Harry's O.W.L. written exam included a question about Polyjuice Potion. Harry felt that he'd done very well with that question, since he'd taken the potion himself a couple of years before (OP31).
• Judging from the Ministry of Magic leaflets on the subject, there appears to be no physical way of telling the drinker of Polyjuice Potion from the original. The only security measure recommended against Polyjuice Potion is to work out a prearranged method of verifying someone's identity, such as a password or a question that only the real person should be able to answer, but this system can of course be defeated by a sufficiently thorough and clever impersonator (HBP3).
• Polyjuice was used by six of Harry's friends in order to become identical decoys as Harry left Privet Drive and flew to the Burrow during the Battle of the Seven Potters. It was actually Snape's plan, but he planted the idea with Mundungus Fletcher in order to make sure the Order of the Phoenix would do it. (DH4, DH33).