(flynetonline)

A West Hollywood police source tells TMZ that when she was arrested, Mischa Barton blew a .12 on her breathalyzer test, smoked marijuana that day (which cops found) and had prescription drugs in her possession - that weren't hers!

Barton was popped at 2:45 AM Thursday in West Hollywood. Cops pulled her over after they witnessed her "straddling two lanes of traffic and failed to signal when making a turn."

She was arrested on suspicion of DUI, possession of narcotics and driving on a suspended license. Barton was taken to the West Hollywood Sheriff's station and was released on $10,000 bail.

Mischa Barton's Mugshot