(think you might've watched the vid already?)
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.
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.
Salvia is classified as the most potent naturally occurring hallucinogen, and can have the following effects:
Uncontrollable laughter
Past memories, such as revisiting places from childhood memory
Sensations of motion, or being pulled or twisted by forces
Visions of membranes, films and various two-dimensional surfaces
Merging with or becoming objects
Overlapping realities, such as the perception of being in several locations at once
From: TMZ
