Miley Cyrus

TMZ has obtained a video of Miley Cyrus appearing to smoke from a bong and incoherently rambling and giggling.

The video, which was shot during a party at Cyrus' home five days after her 18th birthday, shows the pop star taking a hit from a water pipe  and experiencing what she describes as a "little bit of a bad trip." But a source close to the Hannah Montana star tells TMZ that Cyrus is not smoking marijuana, but a natural herb called salvia, which has hallucinogenic properties and is legal in California.

Billy Ray Cyrus and wife file for divorce

Cyrus' father, Billy Ray Cyrus, tweeted an apology to fans within hours of the video's posting. "Sorry guys. I had no idea. Just saw this stuff for the first time myself," Cyrus, 49, wrote. "Im so sad. There is much beyond my control right now."

The "Achy Breaky Heart" singer is in the midst of a divorce from Miley's mother, Tish Cyrus, after 17 years of marriage.

An e-mail to Miley's rep was not immediately returned.

Click here to watch the video.