"The Sorcerer's Stone! Of course -- the Elixir of Life!"
-- Harry in the Forbidden Forest (PS15)
The Elixir of Life extends life, but requires that the drinker consume it regularly for all eternity if the drinker is to maintain his or her immortality (HBP23).
The process of making Elixir of Life is unknown, but is connected to Alchemy and the creation of the Philosopher's Stone.
References from the canon
- Nicolas Flamel and his wife lived over 650 years by drinking Elixir of Life (PS13)
- Voldemort seeks immortality, and stealing the Philosopher's Stone was one step on his quest to attain it (PS15)
- Someone dependent on the Elixir can die if the supply runs out (for instance, if the Philosopher's Stone being used to make it is stolen or destroyed) or if the supply of Elixir is contaminated (HBP23).