Six D.C. physicians and three pharmacists have been convicted in the last two years for illegally distributing prescription drugs -- including heroin substitutes and diet pills. The convictions are part of a nationwide crackdown that prosecutors believe is an important step in stopping the flow of drugs that supply the underground network.

Nearly all the prescription drugs illegally obtained by drug abusers come from legitimate sources, such as physicians and druggists, according to the U.S. Drug Enforcement Administration. In the last year alone, DEA estimates that 300 million dosages of prescription drugs ended up in the illegal market nationwide.

A major factor in the illicit prescriptions drug boom is the tremendous profit potential for the physician and for the dealer. For example, a Dilaudid pill, a heroin substitute popular among drug addicts, costs 17 cents when purchased legally in a drugstore. The same pill sells for between $40 and $60 a pill on the street, according to D.C. police. Physicians in the illegal drug business charge anywhere from $15 to $300 per prescription.

In one six-month period in 1978, one of the physicians convicted issued more than 1,500 prescriptions for Dilaudid that had a street value of more than $2 million, according to Douglas . Behr, an assistant U.S. attorney in charge of the prosecutions.

Many of the physicians prosecuted in the District and around the country have become, in effect, no different from drug pushers -- they sell prescriptions on demand to just about anyone who wants them, Behr said. Illegal drug distribution "is the same offense whether it's committed in an office or on a street corner," Behr said.

But the flagrant drug abuses of some physicians "sometimes blows my mind," said Stephen Stone, a DEA lawyer. "Physicians can be so successful legitimately. The vast majority of physicians (act properly) . . . sBut one bad physician can drive an incredible amount of stuff," Stone said.

In a two-year period, for example, one convicted physician wrote prescriptions for 32,000 Dilaudid tablets. And police investigators found that the same doctor issued 91 percent of the Dilaudid filled by one pharmacy during the period, according to assistant U.S. attorney Joseph F. McSorley.

Dr. Julia D. Brown, a D.C. physician judged mentally incompetent to stand patient eight prescriptions for six different drugs, a total of 500 pills, including Valium, Quaaludes, Preludin -- all on the same day, according to prosecutors. Another District physician convicted on one count of illegal distribution, Dr. Sammye A. Belcher, wrote prescriptions often with little or no physical examination, and in such diverse locations as her kitchen, bedroom and living room, the government said.

And Dr. Pinyon L. Cornish, 79, was convicted of writing a bogus Dilaudid prescription in the name of a patient five days after the same patient died, prosecutors said. (Cornish even signed the man's death certificate, a prosecutor said.)

One convicted pharmacist, Terry M. Watts, 38, was accused of writing prescriptions in the name of a nonexistent doctor, Behr said. The phone number for the fake physician turned out to be a pay phone, Behr added.

Other physicians convicted on illegal drug charges in the probe include a city employe whose job was to examine drug addicts at a District methadone clinic and an assistant professor of pediatrics at Howard University.

Capt. James Nestor, who supervises a special 11-person drug diversion unit of the D.C. police narcotics squad, said investigations of physicians and druggists violating the drug laws can take nearly two years and are difficult to develop. "If a doctor takes a guy's blood pressure and gives a half-way examination, he can beat us," Nestor said.

Nonetheless, prosecutors have obtained convictions in nine of the 10 physician-druggist cases prosecuted in the District in the last two years. Charges wer dismissed against Brown after she was adjudged mentally incompetent to stand trial. The licenses of at least three of the seven physicians prosecuted were revoked. And two physicians each received active prison sentences of six months. Most were given suspended sentences, fined and placed on probation.

Federal regulations provide that physicians must show that the drugs given to a patient are for a legitimate medical need and were prescribed as the result of a doctor-patient relationship, according to DEA lawyer Stone. tA pharmacist, in turn, has a "corresponding responsibility" to make sure that a prescription is for a legitimate medical purpose, Stone said. It is not enough for pharmacists to simply verify that a physician signed prescriptions, Stone said. "If (a druggist) suddenly starts seeing a lot of skinny people coming in for diet pills . . . he can't close his eyes and make believe he doesn't see them," Stone said.

The physicians convicted so far in the drug probe are:

Nahid I. Holum, 43, who was employed in a D.C. government drug clinic. Holum, who is appealing her conviction, is on leave without pay from her $22,672-a-year city job, a city spokesman said. The criminal charges concerned alleged illegal distribution of drugs in her private practice.

Samuel B. Rosser, an assistant pediatric professor at Howard's School of Medicine. The university plans to look into the matter after it receives official notification of the conviction, a spokesman said yesterday.

John B. Fegan, a veteran of more than 30 years of private practice in the District.

Marshall D. Nickerson, convicted on mail fraud and drug distribution charges; as well as Cornish and Belcher. The pharmacists, in addition to Watts, were Rodney L. Eidsness and Eric A. Wynkoop.