When Joseph Wesbecker stormed a printing plant in Louisville, Ky., nearly two years ago, killing eight people and himself with an AK-47 rifle, he unwittingly fired the first shots in another kind of assault.

A coroner's examination revealed that Wesbecker had been taking a prescription anti-depressant drug called Prozac.

The disclosure did not generate much public concern. Wesbecker's doctor had asked him to enter a hospital and to stop taking Prozac, speculating that it might have aggravated his unstable condition.

But there was little reason to think that Prozac, made by Eli Lilly & Co., was harmful. In fact, the U.S. Food and Drug Administration and numerous medical experts said it was safe. About 4 million people take the product worldwide.

For the Church of Scientology, however, the shootings rang out a different message. Long an ardent opponent of psychiatric drugs, the church was particularly opposed to Prozac, a runaway sales success and the first innovation in psychiatric drugs in many years.

Wesbecker's attack was the foundation for a blistering assault launched by the church on Lilly and those who had anything good to say about the drug.

At first, the church provided information to people seeking to sue Lilly over the drug's effects. Church leaders also hit the talk show circuit and fee-based public relations wire services.

Its 20-month campaign rose to unprecedented levels soon after Time magazine ran a highly critical cover story about the church in early May. The church's theory, denied by Lilly and Time, was that Lilly planted the story.

The church bought full-page advertisements in USA Today for two weeks, one week to attack Lilly, the other to attack Time. They were followed by a series of ads last month to promote Scientology. Each series was followed by a glossy booklet insert, one decrying Time and Lilly, the other lauding church founder L. Ron Hubbard and Scientology.

Lilly executives, who at first ignored the Scientology campaign, now express shock at its ferocity and effectiveness. "Take the nastiest negative campaign in politics you have ever seen and cube it," said Mitchell Daniels, Lilly's vice president for corporate affairs and former political director in the Reagan White House.

"A strong case can be made that we should have confronted this from the beginning," Daniels said. But "the real question is, why did they go nuclear?"

The overall campaign, which cost more than $2 million, seems to have been brutally effective. Lilly's Prozac sales have dropped in the United States, and so has its market share.

Heber Jentzsch, president of the Church of Scientology, responded: "They have a multibillion-dollar killer drug on the market, and they want to keep it no matter how many people die on it." Jentzsch said the church believed it "couldn't trust the media to get it right" about its opposition to Lilly and Prozac, so it used "advocacy advertising" to make its point.

Lilly, a multibillion-dollar corporation that has been in the pharmaceutical business for more than a century, invested nearly $250 million in its quest to make Prozac a safe drug approved by federal regulators. Prozac, introduced in early 1988, produced sales of more than $770 million in 1990.

Dr. John Csernansky, a psychiatrist and professor at Washington University in St. Louis, has prescribed Prozac for several patients. Because of the ads, he said, some of his patients are now afraid to take the drug.

"They interpret these newspaper reports as saying these are ordinary people taking Prozac who become raving maniacs, and these are a complete misrepresentation," Csernansky said.

"Suicide is a common, tragic complication of depression. Effective anti- depressants reduce that number (of suicides); they don't increase them."

Before the Scientology campaign, analysts said sales might reach $1 billion this year.

For its part, the church opposes the drug because of the longstanding contempt for psychiatry infused into the creed of Scientology by Hubbard, who died in 1986.

The late science fiction writer and philosopher's best-known work, which describes Scientology's philosophy, is the book Dianetics. Scientologists consider psychiatric drugs destructive, and part of their doctrine is to show no mercy toward an enemy.

But the fight also exposes a dark side of the information era. Well- financed groups such as the Scientologists can buy advertising and televised air time to send information instantly and uncritically around the world. The mere existence and power of such a campaign can overshadow the accuracy of its charges.

Lilly's approach had been to communicate with its customers, most of them physicians, about Prozac's safety, but Scientology's campaign was directed at patients.

Since being introduced into the U.S. market in early 1988, Prozac -- Newsweek called it a "miracle drug" in a cover story -- has been the engine driving Lilly's record profitability. Now, some analysts say, it will never regain its overwhelming popularity. "Prozac will never again regain the sales growth it had before the Scientologists hit," one said.

Lilly has had problems with other drugs. The company pleaded guilty to 25 misdemeanor charges for failing to report to the FDA deaths and illnesses of patients who took its anti-arthritis drug Oraflex.

It also sold, along with other companies, DES, a drug criticized as triggering cancer in women taking it to prevent miscarriage.