Miley Cyrus celebrated her 18th birthday by experimenting with a bong and catching a case of the giggles -- but sources say she was not smoking marijuana.

The video was shot during a party at Miley's L.A. area home 5 days after her 18th birthday.

According to a source connected with Miley ... the smoke filling the bong is a natural herb called salvia which has psychedelic qualities. Possession of salvia is legal in California.

As for the video ... the source tells us it was shot by one of Miley's friends -- and the theory is someone stole or copied the video from that friend's camera.

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