Felix Felicis, also called "'Liquid Luck'", is a magicalpotion first introduced during Professor Slughorn's initial potions class in Hogwarts. Felix Felicis makes the drinker lucky for a period of time, depending on how much is taken, during which everything they attempt will be successful. It is meant to be used sparingly, however, as it causes giddiness, recklessness, and dangerous overconfidence if taken in excess. Felix is highly toxic in large quantities and is also a banned substance in all organised competitions, such as Quidditch, along with all other methods of cheating. It is very difficult to make, disastrous to get wrong, and requires six months to stew before it is ready to be consumed.
Horace Slughorn claimed that he used the potion twice in his life: once when he was 24 years old, and again when he was 57, both of which resulted in a perfect day each.
Harry later pretended to add a small amount to Ron Weasley's drink at breakfast before a Quidditch game so that Ron would feel more confident about his abilities. Hermione noticed what Harry did, and believing that he had actually put something in Ron's drink, warned Ron not to take a drink. He ignored her and drank the pumpkin juice, and he then went on to play a nearly perfect game. Following the game, Hermione went off on Harry for what he had done, and Harry told her and Ron that he had not given Ron the potion at all, and that Ron played that well all by himself.
This potion has a shining gold appearance, resembling molten gold. Small droplets should leap like goldfish above the potion's surface upon completion.
Effects
Horace awards Harry with a bottle of Felix Felicis.
Felix Felicis possibly works by providing the drinker with the best possible scenario. This usually registers in the drinker's mind in the form of an unusual urge to take a certain action, or as a voice telling him to do so. The effectiveness of the potion thus depends on the cooperation of the drinker with the voice, for the drinker may for some reason be unable to follow their Felix-induced urges, or can simply choose not to. This is easily remedied, however, as the potion does not single out only one scenario, but changes paths as the situation unfolds. Then again, the chances of the drinker refusing to follow the urge is very unlikely, as it manifests as a "sensation of infinite opportunity" where the imbiber feels supremely confident enough to be infallible for the duration of the Potion's effect.
Felix Felicis also seem to culminate the magical power of the one who imbibed it, since Harry was able to use refilling charms non-verbally, something he otherwise would have had great difficulty casting.
Limitations
"Luck can only get you so far, Harry... Luck is not powerful enough to get through a powerful incantation"
As Hermione pointed out, the potion is not able to better the chances of the drinker against particularly powerful enchantments, since members of the D.A. were not able to bypass the Peruvian Instant Darkness Powder that Malfoy used.
The potion's effectiveness seems to wane after a certain amount of time, as it did in Harry Potter's situation, when he made his way back to the Gryffindor Common Room and he ran into Peeves, but was only just barely able to dodge him with the potion's influence.
Used the potion to help her escape from Death Eaters' curses
Harry's use
When Harry Potter used Felix Felicis, several events occurred that were beneficial to Harry and his friends; the main being Harry obtaining the memory from Slughorn.
Harry obtained the uncorrupted memory from Horace Slughorn about Horcruxes. During this adventure, evidence of luck was following him.
Harry was able to keep his word to his friend Hagrid and attend Aragog's funeral.
Filch had left the front doors of the castle unlocked.
Slughorn was tempted to accompany Harry to Hagrid's by the possibility of obtaining valuable Acromantula venom.
Harry was able to use a non-verbal refilling spell, even though he had not completely mastered non-verbal spells, so Slughorn would not run out of drink (in order to get him drunk).
Harry bumped into Ginny as they passed each other at the portrait entryway, making Ginny think that her boyfriend Dean Thomas was trying to help her into the portrait hole. This incident, compounded by others, eventually resulted in their breaking up with each other.
Harry also caused the break up of Ron Weasley and Lavender Brown by donning his Cloak of Invisibility before leaving the boys' dormitory. When Ron descended the stairs after him with Hermione Granger, Lavender assumed the worst about the pair of them and dumped Ron.
Harry was able to sneak out and back into the castle undetected by unwanted forces; even when the potion's effect was about to dissipate, he was able to evade Peeves.
The Battle of the Astronomy Tower
Before leaving with Dumbledore to search for one of Voldemort's Horcruxes, Harry gave the rest of the Felix Felicis to Ron and Hermione with the instruction that they were to share it with Ginny, as he believed Draco Malfoy was ready to act on his mission from Voldemort. Ron, Hermione, and Ginny split the potion between themselves and effectively used it in the ensuing battle to dodge the curses sent their way by Death Eaters, many of which were Unforgivables.
Etymology
Felix is the Latin word meaning happy or lucky. 'Felicis' is from the same root, but declined in the genitive case. It is translated as 'of luck.' Therefore, Felix Felicis means 'Luck of Luck', 'Lucky Luck' or 'Luck's Luck.' Also, in latin the words are ennounced by saying its nominative, followed by its genitive.
Also, as the Battle of the Astronomy Tower did not take place in the film, Harry did not provide the Felix Felicis to his friends, and instead, drank the entire bottle for his mission to get the memory.
Slughorn mentions in the film that only a perfect Draught of Living Death will win the prize of Felix Felicis, and that only one person prior to Harry had ever won it. In the book, however, the prize is merely to the person who brews the best potion, and Slughorn even says he does not expect perfection from any of them.
In the video game, Harry fights a duel with Crabbe and Goyle which is unlosable after taking Felix Felicis. Their spells miss you no matter what. When using Depulso to obtain "Mini-Crests", a considerable amount appears from every Depulso target.
Slughorn with the Felix Felicis.
Additionally, the Dragon dung Fertiliser you brew in Potions Club cannot overheat and pouring too much of an ingredient in will have no effect.
