WASHINGTON - The Food and Drug Administration on Wednesday approved a widely used adult psychiatric drug for the treatment of schizophrenia and bipolar disorder in children and adolescents.

The action permits use of Risperdal for schizophrenia in youths aged 13 to 17 and for bipolar disorder for those aged 10 to 17, FDA said.

It was approved last fall for treatment of irritability in autism.

Risperdal, manufactured by Janssen, L.P. of Titusville, N.J., is the No. 3 anti-psychotic drug with $2.3 billion in sales in 2005, according to the pharmaceutical data company IMS Health.

Risperdal was approved for use in adults in 1993.

Until now, FDA said, there has been no approved drug for the treatment of schizophrenia in youths and only lithium is approved for the treatment of bipolar disorder in adolescents.

The dose approved for youths is slightly lower than the adult dose, FDA said.

Drowsiness, fatigue, increase in appetite, anxiety, nausea, dizziness, dry mouth, tremor, and rash were among the most common side effects reported, the agency said.