An Alexandria jury convicted Dr. Joel L. Koslow yesterday of prescribing massive quantities of controlled drugs for nonmedical purposes to a Falls Church woman who died last August of a heroin overdose administered by someone else.

The jury, which deliberated for three hours, recommended a six-month jail term and a $1,000 fine. Sentencing was deferred and Koslow's attorney, Philip J. Hirschkop, said an appeal will be filed.

A spokesman for the State Board of Medicine said yesterday it will meet soon to decide whether Koslow's license to practice medicine should be revoked.

Koslow, 46, a member of the Alexandria Hospital staff since 1971, was indicted last December on charges of prescribing 2,750 doses of drugs including Quaaludes and Percodan -- more than two a day -- for Nancy Moffitt, 33, during the three years before she died.

An Arlington man, Donald J. Munley, 33, of 860 S. Greenbriar St., pleaded guilty to involuntary manslaughter last month after testifying that he gave Moffitt the fatal heroin injection last Aug. 9. Munley is now serving a two-year prison term.

Assistant Commonwealth's Attorney S. Randolph Sengel said in his closing argument that Koslow had told a state investigator he had prescribed addictive drugs for Moffitt because she asked for them and not because she needed them medically.

Sengel told the Circuit Court jury that during the interview with investigator William A. Hurst, Koslow said "he couldn't say no to Moffitt when she asked him for drugs." According to court records, Koslow told Hurst that when he learned of Moffitt's death he felt guilty and destoyed all of her medical records.

Defense attorney Hirschkop said at the start of the trial: "We concede that Dr. Koslow had an extramarital affair" with Moffitt during the three years before her death, "but if you are going to convict Koslow of that, I suspect 90 percent of us would be in jail." Later, Hirschkop said he believed his client was, at most, guilty of "poor practice," rather than a crime.

Hirschkop did not dispute the number of drugs Koslow had prescribed, but argued that they were for legitimate medical purposes. "She had weight problems, she was depressed, she had insomnia . . . she was hospitalized seven times."

Dr. James Cooper, chairman of the Department of Medicine at Fairfax Hospital, testified that "regardless of what medical record" Moffitt had, the prescriptions were inappropriate.

Moffitt, who once worked as a receptionist at the Fairfax County Juvenile and Domestic Relations Court, according to testimony at Koslow's trial, was found dead in her home at 2224 Pimmit Run La., Falls Church, with needle marks on her arms and neck.