Waidan's decline as the dominant branch of Daoist alchemy is documented with unusual clarity, because several of its most prominent patrons were also its most conspicuous casualties. Chinese court records and later historians attribute the deaths of at least five Tang dynasty emperors, including Taizong, one of the dynasty's most capable rulers, and Xianzong and Wuzong later in the dynasty, to poisoning by elixirs containing mercury and other toxic minerals that alchemists had prepared for them on the promise of extending life or conferring immortality, deaths that occurred despite, or in some accounts precisely because of, the intensity of imperial patronage waidan alchemists received during this period.
The pattern repeated across the following Song dynasty as well, and the accumulating record of court alchemists poisoning the very rulers who sponsored them in pursuit of longevity did real damage to waidan's standing and imperial support over the following centuries, a decline compounded by neidan practitioners actively arguing that operations performed within the practitioner's own body were both safer and more spiritually authentic than compounding literal poisons and ingesting them. Waidan never disappeared entirely and its laboratory methods left a substantial mark on the history of Chinese chemistry and pharmacology, some historians treating it as a genuine antecedent of later scientific inquiry into mineral properties, but its central promise, a physical substance that confers deathlessness when swallowed, was undermined by the visible, repeated deaths of the emperors most positioned to prove it true, which is a large part of why neidan's internalised version of the same alchemical vocabulary eventually became the dominant Daoist practice instead.