In "Half-Blood Prince", Harry is invited to the first meeting of the Slug Club held on the Hogwarts Express. Slughorn sets less-stringent entry criteria for Advanced Potions than his predecessor, lowering the required grade from a perfect O (Outstanding) to the above-average E (Exceeds Expectations). This last-minute change enables Harry and Ron to take Potions at NEWT level. However, since Harry had not expected to be allowed to join the course, he has none of the necessary materials, and Slughorn lends him an old textbook until Harry can procure his own. Later, Harry does buy a new copy of Advanced Potion-Making from Flourish and Blotts, but switches the covers and gives back the new book. During his first class, Slughorn offers a small amount of Felix Felicis to the student who brews the best cauldron of the Draught of Living Death. Harry wins with the help of handwritten notes in the borrowed textbook, which, unbeknown to him, had once belonged to Snape. When Ron falls victim to a high-strength love potion, Slughorn administers an antidote, but then nearly kills him with a drink of poisoned mead, not knowing that the bottle had been planted by Draco Malfoy in an attempt to kill Dumbledore. Harry later uses the Felix Felicis to help retrieve a memory from Slughorn that details the conversation between the professor and Riddle about Horcruxes as well as the possibility of creating more than one Horcrux.