More American Youths Abusing Prescription Drugs By SIOBHAN McDONOUGH    WASHINGTON (AP) -- Fewer American youths are using marijuana, LSD and Ecstasy, but more are abusing prescription drugs, says a government report released Thursday.    The 2003 National Survey on Drug Use and Health also found youths and young adults are more aware of the risks of using pot once a month or more frequently...