Last week, an elderly man walked into a Giant pharmacy in Bowie to fill a prescription for the antibiotic tetracycline he had gotten from his dentist. The pharmacist typed the information into the pharmacy computer, called up the man's records and found that he was taking digoxin, a heart medication.

Digoxin and tetracycline don't mix -- the antibiotic increases the action of the heart drug. Side effects could range from mild nausea to a potentially toxic digoxin build-up.

But the pharmacist intervened. He called the man's doctor and dentist and switched the patient to penicillin.

Available behind drugstore counters at most times of the day and night, a pharmacist is among the most accessible of all health professionals. They are not just pill dispensers; they are up-to-the-minute sources of information on the 2,000 prescription drugs likely to be found on a pharmacy shelf and the more than 100,000 over-the-counter medicines.

And pharmacists, like most health professionals, are in the midst of a revolution, caused partly by the explosion in the number and sophistication of drugs, partly by the introduction of computers and partly by a sense that the pharmacists themselves want to be more closely in touch with their patients.

America is a country of pill takers. Pharmacies fill 1.4 billion prescriptions worth $17 billion annually. That works out to about eight prescriptions for each person in the United States each year. Some 75 million U.S. adults take one or more drugs at least once a week. Forty million Americans take over-the-counter preparations every day. About $6.2 billion is spent on OTC drugs each year.

Although most drugs have powerful effects -- both to heal and to harm -- few patients truly understand the directions, the dose they are supposed to take and the side effects for which they must watch.

According to the Food and Drug Administration magazine FDA Consumer, one third to one half of patients fail to take medicines as prescribed. Usually, this means taking the drug at the wrong time of day or in incorrect amounts, skipping doses, or stopping the drug before the course is finished.

And the pills are powerful. For example, a drug like desipramine, one of the tricyclic antidepressants, was a true advance in the treatment of depression when it was introduced more than 15 years ago. But there are problems -- these antidepressants may take several weeks to work; they may cause dry mouth, blurry vision and drowsiness. A patient taking tricyclics may experience reduced sexual desire and ability to have orgasm. Weight gain may occur.

A doctor probably would spend time discussing the possible side effects with a patient, but tricyclics are often prescribed for an extended period of time. After several months, the patient may not remember that his doctor warned him against drinking alcohol while he was taking desipramine. He may forget that the drug interacts badly with certain substances used to treat high blood pressure and asthma.

It's for patients like this, who need complicated drug information, that the pharmacist can be indespensible.

In a nationwide survey done for the FDA in 1983, two out of three patients interviewed within a few weeks of receiving a new prescription said they had received information about their new prescription from their doctor or pharmacist. Only about a third said they had been told about possible side effects of their medication.

More dramatically, the survey showed that very few patients asked questions about the medicine they were getting. Only 2 to 4 percent asked their doctors questions; 3 percent questioned their pharmacists.

Some reasons for this reticence? Exaggerated reverence for the doctor; perception of the pharmacist as a drug clerk rather than a trained professional; anxiety and confusion brought on by the illness; failure to understand the doctor's instructions; lack of time.

Often, patients just don't know what to ask.

In response to this study, the FDA and the National Council on Patient Information and Education developed a list of five questions consumers should ask about prescription drugs:

* What is the name of the drug, and what is it supposed to do?

* How and when do I take it -- and when do I stop using it?

* What food, drinks, activities or other drugs should I avoid while taking it?

* What are the side effects, and what should I do if they occur?

* Is there any written information available about the drug?

It's also a good idea to ask how to store the drug. The light on a sunny window ledge in the kitchen may cause it to deteriorate. The bathroom cabinet may not be a good spot either -- steam from the bath or shower can damage some drugs.

"I think there's a role the pharmacist can fill today that's different from what we've been accustomed to in the last 10 or 15 years," says Leonard J. DeMino, vice president of professional services for Peoples Drug Stores Inc. "In the old days, pharmacists were entrepreneurs. You were the educated person on the block. In the old neighborhoods, everyone came to you for everything.

"As times changed, and neighborhoods broke down, the pharmacy itself changed," DeMino says. "The pharmacist was no longer the recommender of products, but purely the dispenser of products. That relationship that you used to share with patients was lost."

But DeMino, who wears mortar-and-pestle cufflinks, believes the relationship is changing again.

"When I was in school we were taught to refer patients who asked any questions at all to their doctors. Now all that has changed dramatically. Now, the pharmacist is almost more of an information source than the dispenser of a product."

DeMino's son, Joseph, newly graduated from the University of Maryland's School of Pharmacy in Baltimore, is a product of this new emphasis in pharmacology education.

"I used to hang around in the Georgetown Peoples when my father worked there," recalls Joe DeMino, who works at Peoples' Friendship Heights store. "I guess I was about 10 years old. So when I think of pharmacy, I think of this atmosphere." It's a busy Monday morning; one customer waits by the cash register for a prescription she needs to fill before her bus leaves. Another has just gone off to explore the mysteries of the over-the-counter cold remedies section.

"I get a fair number of telephone inquiries," says the 24-year-old pharmacist. "Things like an elderly lady calling to ask if she could have a drink, because it was her birthday, and she was worried about all the pills she takes. Another woman calls to ask about antimalarials and the new motion sickness patches. She was going off to Africa.

"I know in the old days people used to call their pharmacists 'Doc' and have very personal relationships. It's gotten a little impersonal today. We're such a busy society. But even though I'm new at this, I think that old relationship might be starting up again. I hope so -- it's the best part of the job."

Pharmacists learn about physiology, biochemistry, biology and pharmacology. But since drug preparation and packaging has been taken over by pharmaceutical manufacturers, pharmacists increasinglyhave become interested in patient care and education. These days pharmacists' training also includes substantial time devoted to clinical work in both hospital and community settings.

A good pharmacist should be approachable both behind the counter and by telephone. Patients also should ask if emergency prescription service is available.

They can also ask if the pharmacist keeps a patient profile -- a record of all medications the individual is using, and the names, addresses and phone number of the doctors who prescribed them. One physician may not be aware of what another physican has prescribed for the same patient. Potentially troublesome interactions can be prevented if the pharmacist keeps a profile.

Health problems associated with adverse drug interactions cost $4 billion in hospital bills and 100 lives annually, estimates the Boston Collaborative Drug Surveillance Program at Boston University.

Computerizing the pharmacy, however, especially the high-volume retail drugstores, may cut those costs. Computerized patient profile systems designed to keep detailed records and monitor potential drug interactions are becoming commonplace. Some of the machines sound a beeping alarm if two incompatible drugs turn up in one patient's file.

Around the metropolitan area, Giant pharmacies were the first to computerize. Other large chains soon followed suit.

"We started investigating the system around 1980," says Sheldon Pelovitz, manager of pharmacy systems for Giant. Today, there are computers in all 72 Giant pharmacies.

To use the service, customers fill out a form listing medical and drug information, which is stored in the computer. Whether records are ept on file cards or floppy disks, it's important to tell the pharmacist about all the drugs normally taken, including alcohol and tobacco, as well as over-the-counter medicines. It's also advisable to have one pharmacist fill all prescriptions in order to keep the records complete and up-to-date.

The Giant system keeps track of prescription drugs at the ingredient level. "If you're allergic to a particular substance, and it's in the ingredients of your prescription, the computer alerts the pharmacist to that fact," says Pelovitz. The computer also watches for drug interactions. Should a second medication be prescribed, the computer alerts the pharmacist, who can check the severity of the problem.

The computer profile also tracks drugs taken on a regular basis, drugs taken in the past, whether safety caps are needed, whether generic drugs can be substituted, and what insurance company should be billed. Records are kept confidential, but clients may ask to have their prescription records printed out to help make medical claims on their taxes. Best of all, this service is free.

The Giant computer also serves as an education tool: With each prescription, the patients receive a 3-by-5 print-out about the drug, listing its uses and possible side effects, and detailing how it should be taken. The handouts are similar to Patient Medication Instruction sheets published by the American Medical Association and available from many doctors.

The Giant computer data base also encom-passes limited information on some common over-the-counter drugs like antihistamines, antacids and painkillers.

While most consumers know that alcohol and drugs don't mix, few people know that food can affect drug action, too. Foods can make drugs work faster or slower, or even prevent them from working at all. The calcium in dairy products, for instance, impairs absorption of the antibiotic tetracycline. But the ascorbic acid in citrus juices speeds the absorption of iron from iron supplements.

Foods traditionally considered "good for you" can cause problems when they interact with certain drugs. Eating large amounts of liver or leafy vegetables can interfere with the effectiveness of anticoagulants, because the vitamin K in the food promotes clotting. People taking the class of antidepressants known as MAO (monoamine oxidase) inhibitors should be aware that such foods as yogurt, chicken liver, avocados and bananas -- all of which contain thiamine -- may cause their blood pressure to rise to dangerous levels.

Soon, Pelovitz hopes, detailed information on food and drug interactions will be part of the Giant pharmacist's computer data base. Giant may eventually be able to sell food, sell drugs, and tell the customer how the two could interact in the body.

The computers also help pharmacists have more time to talk with their customers, and they like that. Two papers presented at the annual convention of the American Pharmaceutical Association, held last month in San Antonio, Tex., suggest that the desire for more consulting time is widespread among pharmacists.

At the convention, pharmacists from the University of Michigan and Wayne State University reported on a survey of 220 community pharmacists in Michigan. The respondents reported that they advised an average of 29 patients each day about medication. About half of those given advice came in for new prescriptions, and about a fifth were getting refills. Time constraints, however, prevented more extensive consultations.

The pharmacists were more likely to talk to patients about such classes of drugs as anticoagulants (which thin the blood) and antibiotics than such products as bronchodilators (which widen the passages in the lungs) for asthma or oral contraceptives.

Pharmacists at the University of Florida in Gainesville surveyed pharmacists' job satisfaction and found that they wanted to spend 25 percent of their time consulting patients, rather than the estimated 11 percent they now devote to it.

One local pharmacist solved the problem of finding enough time to talk with patients: She made a business of it. Beatrice Adreon runs Pharmacy Counseling Services Inc. in Arlington.

In 1973, after nearly two decades of pharmacy work, she entered George Washington University to study counseling, management, women's studies and human behavior.

While at George Washington, Adreon worked as a consultant to the university's gerontology program. She learned that 95 percent of the elderly in this country are not in institutions -- and yet an overwhelming number of them take a complex set of prescriptions medicines every day.

She also learned that while in 1929, 70 percent of diseases were acute conditions like pneumonia, diphtheria and tuberculosis, acute conditions had dropped to only 20 percent by the 1980s. Today's diseases are chronic and often require long-term medication.

Adreon decided to offer in-depth monitoring for patients taking complex prescription drug regimens. She opened Pharmacy Counseling in 1978, to help people who take two or more prescription or over-the-counter drugs.

She compiles a six-page medical and drug profile of her clients in a two-hour initial consultation, and schedules follow-up sessions as appropriate. And she makes house calls.

Adreon sees her service as marriage of counseling with pharmaceutical knowledge. "I get to know the patient's personal history, their heritage, their daily habits. I see what I do as coordinating book knowledge with knowledge of the individual person."

But the service is expensive: $150 for the initial two-hour consultation, and $40 an hour after that. There are variations in this fee schedule, however, depending on individual cases. So far, insurance programs don't cover the service.

Adreon has spent a fair amount of time speaking at pharmacy schools around the country and believes that many of the younger pharmacists she meets find the idea of a profession like hers appealing.

"It's catching on like wildfire. These younger people are coming out of an intense clinical training experience, and they're anxious to use those skills," she says.

Last year, Adreon took part in a think tank conference with a group of other leaders in the profession. For three days, they discussed pharmacies of the 21st century.

"Changes are coming," she says. "Before long, pharmacies will be more like community health centers with group practice. They'll offer more than medications -- they'll be places where people can go to get the blood levels of their medications measured, where conditions can be monitored. And this will be offered on a neighborhood level, as a kind of 21st-century corner drugstore."