Why did Dr. Thurman prescribe placebos to Donnie?
Dr. Thurman is trying to help Donnie Darko without putting him on medication which could possibly have negative side effects.
Furthermore, Dr. Thurman who is being manipulated by the beings of the future appears to know that there is something special about Donnie and is one of several people helping him fulfill his true destiny. However — she is not mentioned in the list of Manipulated Living at the back of the Philosophy of Time Travel.
Also, water is the element through which Frank reaches Donnie; when Donnie takes his placebos, he facilitates a connection with his messenger.
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