ANSAS CITY, Mo., Aug. 17 — In the relatively sedate and step-by- step world of the retail pharmacist, Robert R. Courtney was considered something of a swashbuckler, a wealthy, energetic businessman, adept at catching the next entrepreneurial wave.

One of Mr. Courtney's more enterprising moves, fellow pharmacists say, involved chemotherapy drugs. Some 20 years ago, he became one of the first pharmacists in Kansas City to dispense the cancer medication in premixed bags to doctors. That saved doctors the trouble of mixing and measuring the drugs and allowed them to hook the bags directly up to patients' intravenous tubes.

"A lot of pharmacists don't want to do that, they don't want the responsibility," said Jim Frederich, a retired pharmacist who employed Mr. Courtney for a decade and then sold him his pharmacy. "He was always looking for some other avenue to provide a service, always thinking outside the box."

But federal authorities now believe that Mr. Courtney's zeal may have had another, potentially devastating, side effect.

On Wednesday, the pharmacist was charged with diluting the cancer medicine he dispensed. Federal investigators say hundreds of cancer patients may have received treatment that was drastically weaker than what their doctors had intended.

Mr. Courtney, a man worth more than $10 million, told investigators he cut the drugs' strength "out of greed," according to court papers filed today by prosecutors. They said he admitted he began diluting drug potency in November 2000 and stepped up the deception from March through May of this year.

"It's an astonishing case even to the people who are working it because it involves a very egregious breach of trust between a pharmacist and a doctor and patient," said Jeff Lanza, a spokesman for the Federal Bureau of Investigation in Kansas City. He said investigators believe that at least 10 physicians could have received diluted drugs. Investigators have been trying to contact patients who may have received diluted drugs and are checking records from Mr. Courtney's business, Research Medical Tower Pharmacy.

Mr. Courtney is accused of diluting Taxol and Gemzar, used to treat a variety of cancers, including pancreatic and lung cancer, advanced ovarian and breast cancer, and AIDS- related Kaposi's sarcoma. The filing also said Mr. Courtney admitted diluting two other chemotherapy drugs, Paraplatin and Platinol.

Federal investigators say that samples of Taxol and Gemzar that Mr. Courtney dispensed contained no more than 39 percent of the medicine that had been prescribed. One premixed bag contained less than 1 percent of the Gemzar prescribed. The drugs are usually dissolved in saline, and Mr. Courtney was apparently putting higher proportions of saline into the bag.

Such discrepancies could have netted the pharmacist a substantial amount of money, a federal affidavit in the case suggests. In one instance, Mr. Courtney is accused of providing only 450 milligrams of Gemzar for what should have been a 1,900 milligram prescription. The price of 1,900 milligrams is $1,021, compared with $242 for 450 milligrams. Consequently, Mr. Courtney could have pocketed $779, the affidavit says.

Medical and pharmaceutical experts say they have never heard of another case in which a pharmacist was accused of diluting medication.

"Everybody I've talked to is just unbelieving aghast, and just can't believe this kind of thing could happen in the United States," said Dr. Fred DeFeo, chairman of council of the Missouri State Medical Association. "It is certainly possible that some have had cancers that could have been cured that weren't."

A lawyer for Mr. Courtney, Jean Paul Bradshaw, said that his client would plead not guilty and would not comment on Friday's filing. In an interview, Mr. Bradshaw said Mr. Courtney was cooperating with investigators. While not acknowledging wrongdoing on the part of his client, Mr. Bradshaw said that he believed that no more than 50 patients could have been affected and that only one physician was affected. In the court filing, Mr. Courtney also said the dilutions affected only one doctor's patients and that the doctor was unaware of the tampering.

On Wednesday, a federal magistrate, saying that Mr. Courtney was a flight risk, ordered him held without bail on a single charge of adulterating and misbranding a drug. It carries a maximum penalty of three years in prison and a fine of $250,000.

The F.B.I. has said that homicide or manslaughter charges were possible if investigators linked the diluted drugs to a death.

More than 1,000 people have called an F.B.I. hot line, and two civil suits have been filed. Many people say they are agonizing over the prospect that they or their loved ones may have been deceived.

"The thought of a person doing that is devastating — to shorten a person's life even one minute is too much," said William Van Sant, 56, of Independence, Mo., whose wife, Rachel, received Taxol from Mr. Courtney's pharmacy to treat her endometrial cancer. Mrs. Van Sant died last year after her three chemotherapy treatments were unsuccessful.

Mr. Van Sant met this week with his wife's oncologist. "I asked her if she could have received a diluted solution," Mr. Van Sant said, "and she said it could have been possible."

Mrs. Van Sant's doctor, Verda Hunter, alerted the F.B.I. to the possibility that Mr. Courtney was diluting drugs, said Diana Jordison, a spokeswoman for Dr. Hunter.

The federal affidavit, which does not name Dr. Hunter, said the doctor was told in May by a representative from the Eli Lilly Corporation, the sole manufacturer of Gemzar, that records showed that Mr. Courtney's store could not have been supplying the doctor with as much Gemzar as she thought she was getting. The representative said Eli Lilly's records showed Mr. Courtney had purchased only about a third of the amount he claimed to have supplied.

The doctor then sent samples of drugs she had ordered from the pharmacy in for testing. When the laboratory reported those drugs were at only one-third strength, she called federal authorities. Investigators asked her to order more Taxol and Gemzar, and, said Chris Whitley, a spokesman for the United States attorney's office here, "all of those so far have shown to contain considerably less medicine than the doctor had prescribed and than what the pharmacy had labeled these prescriptions as containing."

Mr. Lanza said that because the specific doses that patients were prescribed are most likely gone, it may not be possible to prove that an individual patient received diluted medicine. He said investigators might have to rely on "a lot of circumstantial facts," comparing drugs prescribed to the amount Mr. Courtney purchased from manufacturers and showing "that it would be physically impossible for that person to receive the prescribed amount of drug."

Some pharmacists who are friendly with Mr. Courtney have suggested that he might have been working with drugs that were diluted in the manufacturing or were improperly labeled. Several oncology experts disagreed, saying drug production is tightly regulated.

Oncology experts said most major cancer clinics and many smaller oncology practices make sure they mix drugs themselves to avoid mistakes in the prescribed dosage.

"I'd want to deal with a rep that I could look at in the eye and say `these are your drugs, right?' and be confident in the quality," said Dr. Nicholas J. Vogelzang of the University of Chicago Cancer Research Center.

In fact, in Kansas City, John Hennessy, executive director of one oncology and hematology clinic, said in an interview that two of his clinic's doctors had left another clinic partly because that clinic did not mix its own chemotherapy drugs.

At his hearing, there were several supporters of Mr. Courtney, 48, the son of a retired Assemblies of God minister. A 1975 graduate of the University of Missouri at Kansas City, he is married and has five children.

Some cancer experts said today that giving diluted medicine to a cancer patient is galling because patients with such a desperate illness are so reliant on their doctors.

"For cancer patients who are so dependent on us in their fight against their disease," said Dr. John E. Niederhuber of the Association of American Cancer Institutes, "you ask how could this happen?"