"He [Harry] thought his voice might soon vanish from lack of use. He bent a little lower over Advanced Potion-Making and continued to make notes on Everlasting Elixirs, occasionally pausing to decipher the prince's useful additions to Libatius Borage's text."

—Harry tries desperately to ignore Hermione's ranting about Ron while doing his homework
Everlasting Elixirs are a type of potion with the effect to either never run out of potion or to work forever.

Harry Potter studied Everlasting Elixirs in sixth year Potions class, taking notes on them in the Hogwarts Library from his copy of Advanced Potion-Making, which the Half-Blood Prince had annotated.

Unknown Brewing Instructions