The Hourai Elixir was created by Eirin Yagokoro using Kaguya Houraisan's power over eternity. Anyone who drinks this forbidden elixir will cease to age, never become ill, and will never be able to die. Every injury shall cease; critical injuries will be healed in a matter of days. Complete destruction results in complete restoration.
Drinkers gain the inability to die, allowing their bodies to receive an indefinite amount of punishment. No matter whether they are beheaded, incinerated or has taken any other form of damage, they will always revive without exception. They will recover within a few hours if they take a non-fatal blow. Due to the nature of the Hourai Elixir drinkers are also incapable of contracting any form of disease or illness. This includes the natural process of aging, they needs no sustenance such as food, water, air, warmth or sleep to stay alive. Every injury shall cease; critical injuries will be healed in a matter of days. Complete destruction results in immediate and complete restoration. The Hourai Elixir is permanent, their immortality is irreversible and cannot be copied or interfered with, they will always be restored to pinnacle condition. According to Fujiwara no Mokou, the first taste of the medicine makes the drinker incapable of growing up or aging. The second taste cures all imperfections, granting perfect physiology and makes them incapable of becoming ill. The third taste makes one completely immortal to the truest sense. Drinkers become an existence completely independent of even the concept of reality, thus making one not subject to the bounds of life, death, or even manipulation. (nourishment such as food, water, or sleep does not impact her body at all), This makes them completely indestructible, and it extends to their mental and spiritual states just as much as their physical state, as shown with Mokou, Kaguya, and Eirin who are all shown living a carefree life free of mental exhaustion even after 1300 years. They will never be able to die.
The Hourai Elixir is permanent. According to Mokou, the first taste of the medicine makes the drinker incapable of growing up or aging. The second taste makes them incapable of becoming ill. The third taste makes one completely immortal to the truest sense. The ingredients of the Hourai Elixir are known only by Eirin Yagokoro, whose name points her out to be the "Mind of God", an omniscient being. Because she made the Hourai Elixir the true elixir of immortality, she probably also made it irreversible, making its effects perfectly permanent. In her confrontation with Mokou, Alice points out that the soul is made invincible. This, along with Yuyuko's and Yukari's scenario in Imperishable Night, makes it seem that the Hourai Elixir itself isn't a power that keeps one immortal, but makes one an existence completely independent of even the concept of reality, thus making one not subject to the bounds of life, death, or even manipulation, although the pain of injuries can still be felt. This medicine indeed makes one truly invincible, and forced to live out their existence forever. It's no wonder why this medicine is forbidden even to the mysterious lunarians.
Since this elixir is forbidden, and only Eirin is known to be capable of manufacturing it—and even then only with the help of Kaguya—it is understandably rare. Only four people are confirmed to have ever drunk it: Chang'e, Kaguya, Fujiwara no Mokou and Eirin herself.
■Lunarians have very long lifespans, but only by remaining free of impurity such as life on Earth. Escaping such impurities is why the the capital on the moon was created in the first place. Just as Reimu can negate it by introducing the tainted life of her home planet, the benefits are only in effect so long as the inhabitants stay in the lunar capital. Eirin would not have the benefit of her Lunarian lifespan on Earth. Choosing to follow her princess in eternal exile, she would have had to take the Hourai Elixir.
■The Ghost Team's ending in Imperishable Night reveals that Eirin herself has consumed the Hourai Elixir (lost in translation, especially due to not being stated directly). The gist of the conversation is that she has taken medicine prepared through Kaguya's power and therefor can never become a resident of the Netherworld. Yuyuko is taken back, claiming it makes Eirin a "natural enemy." Along with Kaguya and Fujiwara no Mokou, she is among the few immune to Yuyuko's power of death.
■Eirin is one of the few characters that revive (along with Yuyuko, Kaguya, Tenshi and Suwako) after being defeated the first time.
■Inaba of the Moon and Inaba of the Earth contains a contradiction when Eirin becomes sick. This should be impossible for an immortal who has taken the Elixir. On the other hand, the series is a light-hearted comedy not written by ZUN and not exactly canon.
History of the Hourai Elixir
The backstory of the Hourai Elixir is that Eirin made the medicine on Kaguya's request to drink it, resulting in her exile from the Moon. According to Cage in Lunatic Runagate: Chapter 1, Eirin felt guilt over Kaguya's exile, and when the time came to bring Kaguya back to the Moon, Eirin deceived the Moon's emissaries and fled back to the Earth with Kaguya. With this tied to the traditional story of Kaguya-hime, Kaguya apparently left the Hourai Elixir to the old bamboo cutter who raised her and the emperor whose heart she'd stolen before she left to the Moon, resulting in Fujiwara no Mokou's backstory in Cage in Lunatic Runagate: Chapter 4. There is no information as to where the Hourai Elixir is presently or even if there is any left.
Reportedly, the ingredients of the elixir remain in the liver of whoever drinks it. Eating the raw liver of an immortal is said to grant the same immortality as drinking the elixir, though it will not deprive one from their own because they have already been rendered deathless.