ASHINGTON, Aug. 25 — As demand for prescription drugs rises, pharmacies in the United States face a shortage of trained professionals, increasing the risk of improper medication, pharmacists and health care experts say.

About 6,500, or 6 percent, of the pharmacist jobs at chain pharmacies are unfilled, says Phillip L. Schneider, a vice president at the National Association of Chain Drug Stores. That is an increase of almost 4,000 since February 1998. In hospitals, about 21 percent, or 12,600, of the pharmacist jobs are unfilled, according to a survey in June by the American Hospital Association.

To fill the vacancies, pharmacies are offering higher salaries. From March to September last year, the average starting salary for a hospital pharmacist with little or no experience jumped to $60,740 from $56,000, an increase of 8.5 percent, according to the Department of Health and Human Services.

Despite the rising salaries, pharmacists say they expect the shortage to get worse in the next few years. The main concern is that errors will increase as pharmacists become increasingly overworked.

"The pharmacist shortage is a prescription for increasing medication errors," said Mary Anne Koda- Kimble, dean of the School of Pharmacy at the University of California in San Francisco.

Another concern is that the shortage may cause pharmacies to pull out of some areas, particularly rural areas. Some have already begun cutting hours and eliminating weekend service.

Medication errors cost the health care system more than $177 billion last year, up from $77 billion in 1995, according to the Journal of the American Pharmaceutical Association. And the Health and Human Services Department says that errors also cause more than 7,000 deaths a year.

But, said Carmen Catizone, the executive director of the National Association of Boards of Pharmacy, which represents state pharmacy boards and works to strengthen safety standards, it is very difficult to determine how many of the errors are caused by pharmacists.

"When medication errors occur in pharmacies," Mr. Catizone, said, "because of the liability, we don't get the information."

He said that despite persistent lobbying efforts by his organization, only one state, North Carolina, had passed a law requiring pharmacies to report medication errors.

The shortage of pharmacists also hinders their ability to catch errors made by others. "The pharmacist often represents the last safety net," Dr. Koda-Kimble said.

The shortage has been caused largely by increasing demand for prescription drugs in the last decade or so as the population has aged and health care plans offering prescription drug coverage have expanded.

In 1990, 2 billion prescriptions were filled in the United States. By last year, the number had jumped to 3.15 billion, Mr. Schneider said.

As the demand for pharmacists rose in the 1990's, pharmacy school enrollments dropped every year beginning in 1994, said Will Lang, a spokesman for the American Association of Colleges of Pharmacy. Many licensed pharmacists were also lured away to jobs produced during the decade's economic expansion.

The number of pharmacists has risen slightly, to about 196,000 today from almost 181,000 in 1995, according to the Department of Health and Human Services. But the growth has not been nearly enough.

The increased demand for drugs has meant more administrative work for pharmacists, who on average spend as much as 20 percent of their making sure prescriptions are covered by health plans and explaining limitations to patients.

"Pharmacists need to be able to spend less time explaining insurance plans and more time explaining medication," said Susan Winckler, a policy director at the American Pharmaceutical Association.

Congress has taken notice of the pharmacist shortage. Legislation has been introduced in the House to help broaden enrollment at the 83 pharmacy schools. But some say that much more must be done.

"Major systematic changes will be needed," Dr. Koda-Kimble said, or "the pharmacist shortage is likely to become more acute."