The Food and Drug Administration is warning U.S. consumers that the vast majority of Internet pharmacies are fraudulent and likely are selling counterfeit drugs that could harm them.

The agency on Friday launched a national campaign, called BeSafeRx, to alert the public to the danger, amid evidence that more people are shopping for their medicine online, looking for savings and convenience.

Instead, they're likely to get fake drugs that are contaminated, are past their expiration date or contain no active ingredient, the wrong amount of active ingredient or even toxic substances such as arsenic and rat poison. They could sicken or kill people, cause them to develop a resistance to their real medicine, cause new side effects or trigger harmful interactions with other medications being taken.

"Our goal is to increase awareness," FDA Commissioner Dr. Margaret Hamburg told The Associated Press, "not to scare people away from online pharmacies. We want them to use appropriate pharmacies."

That means pharmacies that are located in the U.S., are licensed by the pharmacy board in the patient's state and have a licensed pharmacist available to answer questions. In addition, the pharmacy must require a valid doctor's prescription for the medicine. Online drugstores that claim none is needed, or that the site's doctor can write a prescription after the customer answers some questions, are breaking the law.

Research by the National Association of Boards of Pharmacy, which represents the state pharmacy boards, found that of thousands of online pharmacies it reviewed, only about 3 percent follow state and federal laws. In fact, the group's website lists only a few dozen Internet pharmacies that it has verified are legitimate and following the rules.

Most consumers don't know that. An Internet survey, conducted by the FDA in May, questioned 6,090 adults. It found that nearly one in four Internet shoppers has bought prescription drugs online, and nearly three in 10 said they weren't confident they could do so safely.

The campaign comes after some high-profile cases of counterfeit drugs reaching American patients earlier this year.

In February and again in April, the FDA warned doctors and cancer clinics around the country that it had determined they had bought fake Avastin, a pricey injectable cancer medicine, from a "gray market" wholesaler. The fake Avastin vials originated in Asia or Eastern Europe and were transferred through a network of shady wholesalers before being sold to clinics by a wholesaler claiming to be in Montana.

In another case, the FDA issued a warning in May after learning consumers shopping on the Internet had bought fake versions of generic Adderall, a popular medication for attention deficit hyperactivity disorder.

No deaths or serious injuries have been linked to those fakes, but Hamburg notes that when drugs don't help patients get better, doctors usually blame the disease or assume a different medicine is needed. That means most fakes aren't detected.

So the FDA, which has put increasing focus on the counterfeiting problem, on Friday launched a website, www.FDA.gov/BeSafeRx , that shows consumers how to determine if an online pharmacy is safe.

"Buying prescription medicine from a fake online pharmacy can be dangerous, or even deadly," the site warns.

It includes tips on how to spot illegal pharmacies, links to state databases of licensed pharmacies and explanations of all the dangers of rogue pharmacies. Besides likely getting fake drugs, that includes the risk that they will infect your computer with viruses, sell your personal and financial information to other rogue websites and Internet scammers, or charge you for products you never ordered or received.

Many rogue pharmacies claim to be in Canada — because Americans know medicines are cheaper there and assume that's why they're getting a deal. Many fraudulent sites even put the word Canada in their name, or display the Canadian flag prominently on the site. Their web storefronts are slick and look professional. And they all offer prices that are unbelievably low.

"If the low prices seem too good to be true, they probably are,' Hamburg said.

The FDA is collaborating with several other federal agencies and departments and even Interpol in the campaign, Hamburg said, and it has asked medical and pharmaceutical industry groups to join in.

It's also reaching out to doctors, pharmacists and medical facilities to spread the word. They'll get access to materials they can download, from patient fact sheets and discussion guides to sample blog items and web banners for a practice's own website. There's also a list of tips to help doctors determine if a patient may be buying medicine online.

The agency will do a follow-up survey to see if the campaign's message is reaching the public.

"What's truly important to us is that consumers know how to look for an online pharmacy that's legitimate and safe," Hamburg said.