The Hannah Montana star, who turned 18 years old last month (Nov10), has been caught on camera puffing on a huge pipe days after celebrating her birthday.
In the video, obtained by TMZ.com, the star is seen inhaling huge lungfuls of smoke, allegedly from the burning of controversial but legal psychoactive plant Salvia.
She then giggles uncontrollably and asks pals if she's "tripping" - before mistakenly identifying a mystery man nearby as her on/off boyfriend Liam Hemsworth.
In the film, shot by a female friend of the singer/actress, a pal is initially seen lighting the pipe and telling Cyrus to "Hold it! Hold it!" as she takes in the smoke.
Cyrus then flops back in her chair and laughs uncontrollably before telling her friends: "Oh I'm gonna lose it now... I'm going. Having a little bit of a bad trip! Is that a f**king Liam lookalike or what the hell is that? Is that my boyfriend? Is that my boyfriend? Oh my God, he looks so much like Liam!"
The teen star points to her head and giggles as she asks a pal: "Is it me tripping? Is it me tripping?" before breaking down in uncontrollable laughter.
She then asks the woman who is filming her antics at the party: "Oh my God, are you being as weird as I think you are right now?"
As the clip ends, Cyrus cackles loudly again and starts babbling incoherently, before the woman holding the camera tells her: "You are going to s**t a brick when you see this."
Although the Salvia plant is legal in California, it was banned in Delaware in 2006 following the suicide of teenager Brett Chidester, after his parents argued the substance played a significant factor in his death.