In the video, Cyrus takes a deep drag off of a glass water pipe, then reclines on the couch, saying "Okay, I'm going to lose it now. Having a little bit of a bad trip."
A source connected to Cyrus said the smoke filling the bong is from a natural herb called salvia which has psychedelic qualities, TMZ reports.
Possession of salvia is legal in California and several other states. Salvia goes by nicknames including Sally-D and Magic Mint. It is a hallucinogen, but unlike hallucinogens like LSD or PCP, salvia's effects last for a shorter time.
The source told TMZ that the video was shot by one of Cyrus' friends, and that someone may have stolen or copied the video from that friend's camera.
In the video, Cyrus laughs nonstop, while talking about a fellow partygoer who resembles her boyfriend, Liam Hemsworth.
"Oh my god, is that a f**king Liam lookalike, or what the hell is that?" she says. "Is that my boyfriend?"
Behind her, a person whose face is digitally smudged eats Frosted Flakes out of the box while other partygoers come and go.
"Is that me tripping? Is that me tripping? He doesn't look like Liam at all?" she asks later in the video before rolling around in laughter.
Cyrus has taken heat in the past for dancing suggestively with an older man when she was 17, and dressing provocatively at her concerts, attended largely by preteen girls.