Think you need antibiotics to fight that cough or cold? Numerous Web sites are willing to sell them to you without a doctor's prescription -- a loophole, researchers say, that could undermine efforts to curb the problem of bacteria that shrug off powerful antibiotics.

In a simple Internet search, investigators found 138 online vendors that sell antibiotics without a doctor's prescription. More than one third supplied the drugs with no questions asked, while 64 percent made their own prescriptions after having prospective customers fill out an online health survey.

The problem, the researchers report in the Annals of Family Medicine, is that these antibiotics are likely to be used inappropriately.

And that, in turn, could contribute to the major and growing problem of antibiotic resistance -- where populations of bacteria become immune to the drugs that once controlled them.

Antibiotic resistance is a public-health concern worldwide, as bacteria that cause skin infections, meningitis and pneumonia, to name a few, have developed immunity to certain antibiotics.

Misuse of antibiotics -- using them for infections like the common cold that are caused by viruses, not bacteria, for example -- has helped fuel antibiotic resistance. So far, efforts to address the problem have mainly focused on changing doctors' prescribing practices.

But if people are diagnosing and treating themselves with the help of online vendors, then simply altering prescribing practices may not be enough, according to Dr. Arch G. Mainous, the lead researcher on the new study.

In an interview, he noted that many people may, after having a bacterial infection treated with antibiotics, feel they can recognize and treat the next one. Seeing the doctor may seem like a waste of time and money.

Patients feel like they know what's wrong and what they need to take, said Mainous, of the Medical University of South Carolina in Charleston. But I would contend that that's probably inaccurate.

In addition to feeding the wide-scale problem of antibiotic resistance, taking the drugs without a prescription also presents more-immediate potential risks, Mainous noted -- like allergic reactions or interactions with other medications a person may be taking.

It could also well be a waste of money, he pointed out. While antibiotics work only against bacterial infections, many people may buy them for illnesses against which the drugs are ineffective -- like colds and coughs caused by viruses.

In addition, antibiotics are not one-size-fits-all; some are broad- spectrum, meaning they work against a range of bacteria, while others are effective only against certain bugs.

People who buy online without a prescription are picking the antibiotic on their own, Mainous noted. And they're probably just picking one they've heard of, he said.

It is illegal in the U.S. to sell prescription drugs without a valid prescription. But, Mainous and his colleagues write, some online vendors try to circumvent the law by providing their own diagnoses and prescriptions -- as many in this study did.

Others market to U.S. customers but operate in other countries. The online vendors in this study operated out of various locations, including Mexico, Cyprus and Gibraltar.

SOURCE: Annals of Family Medicine, September/October 2009.