A Woodbridge doctor was charged yesterday with dispensing much-needed pain medication to a female patient only after she performed sex acts with him, federal authorities said.

Abhay Vivek Aggroia "sexually exploited" the patient, who had chronic back and neck pain from an automobile accident, according to a criminal complaint and affidavit filed in U.S. District Court in Alexandria. The court documents say Aggroia, who specializes in internal medicine, fondled the woman and then prescribed Percocet pills for her during a series of office visits starting last year. The behavior is alleged to have continued until January.

Federal officials said they expect Aggroia, 51, to turn himself in today and to appear in court on a charge of unlawful distribution of a controlled substance. Reached at his office yesterday, Aggroia referred calls to his attorney, who declined to comment.

The court papers say Aggroia had signed a statement admitting to "inappropriate physical contact" with the patient.

Aggroia is the second doctor charged in a broad federal investigation into physicians, pharmacists and patients suspected of distributing Oxycontin and other potent narcotics. The other doctor, pain management specialist William E. Hurwitz, was convicted in December of running a drug conspiracy out of his McLean office. He was sentenced to 25 years in prison.

More than 50 people have been convicted in the investigation, which is known as "Operation Cotton Candy.''