When scientist Alexander Fleming transformed green moldy bread into the lifesaving drug penicillin in 1929, he couldn't foresee that Americans eventually would spend $15 billion a year on his discovery.

Fleming probably also couldn't have predicted that more than half of the 100 million antibiotics prescriptions that office-based physicians write annually are completely unnecessary - prescribed for colds and other viral infections for which antibiotics just don't work.

"Doctors say they know they shouldn't do it," said Christine Teague, a clinical pharmacist and infectious disease specialist at Charleston Area Medical Center. "But it's hard when there's a screaming child and a worried parent. They'll go somewhere else. "The public is as much to blame. They expect it."

When antibiotics are taken liberally, they become less potent. Eventually, the bacteria outsmart the medicine, making it ineffective. Then the microorganism resists the antibiotic.

"There's a growing problem in the community," Teague said, noting that the resistance problems began in hospital settings. "Upward of 50 percent of the strep pneumonia bug, one of the most common causes of respiratory infections, now is resistant to some of the most commonly used antibiotics."

Patients make things worse by demanding antibiotics, by not taking the entire number of pills and by borrowing another person's medicine.

In theory, antibiotics destroy or inhibit bacterial cells without harming the host, or the person who harbors the bug. However, many antimicrobials - which include antifungals, antivirals and antiprotozoals - may have some toxicity to humans.

That means patients may experience diarrhea, nausea, abdominal pain, dizziness, vaginal discharge, increased sensitivity to the sun or an inefficiency of birth control pills.

In some cases, people have allergic reactions to certain antibiotics or classes of antibiotics. They may get hives, have difficulty breathing or experience swollen lips and eyes.

Still, antibiotics are popular.

In ranking all drugs by dollar volume of new prescriptions, antibiotics were listed seven times in the top 15 drugs last year by Pharmacy Times magazine. They're mentioned 10 times in the most popular 30 prescription drugs.

>From her standpoint, Teague said some of the most popular antibiotics are Zithromax, Amoxicillin, Augmentin, Levaquin and Ceftin. The last two often are prescribed in hospital settings and can be administered intravenously.

"Penicillin is not routinely used," Teague said. "Some still is used for strep. Amoxicillin has pretty much the same activity as penicillin."

Oral antibiotics generally fall into one of six groupings: penicillins, cephalosporins, sulfa drugs, tetracyclines, macrolides and fluoroquinolones. They treat a variety of infections from upper respiratory problems to salmonella and from gonorrhea to chlamydia.

"Respiratory problems are the most common cause to need them," Teague said.

But about half the antibiotics in the United States are used in the production of food animals, according to the American College of Physicians and American Society of Internal Medicine. Most of those, or 90 percent, are given for growth promotion or disease prevention, not for infections.

Maybe that's why pharmacist Gina Rohal said antibiotics often are the size of horse pills.

Rohal, who works for Drug Emporium, said sales increase in the fall when children return to school.

Rohal believes Zithromax is the most popular type of antibiotic. Others in high demand are Amoxicillin, Bactrim, Cipro, Biaxin, Doxycycline, Keflex, Penicillin, Erythromycin and Levaquin. Generic brands are available for six of these.

"We've definitely seen a rise in Cipro sales," Rohal said. Cipro is the front- line drug used to fight anthrax.

For purposes of control and proper usage, all antibiotics require a prescription signed by a physician or physician's assistant. Still, they are misused, Teague said.

In a 1999 subcommittee hearing on the problems of antimicrobial resistance, Sen. Bill Frist, R-Tenn. and a physician, said $30 billion is spent on the cumulative effects of antimicrobial resistance each year.

And hospital-acquired infections caused by antibiotic-resistant bacteria cost $1.3 billion each year.

"There is no other class of medicine that affects the public health as much as antibiotics do," Teague said. "It is important to use them correctly."