Story highlights Sites shut down for selling counterfeit or substandard medication, FDA says

Officials arrested 58 people and seized more than $41 million worth of illegal medicines

Many websites had names that could be confused with legitimate pharmacy retailers

The prices may look tempting, but ordering from an online pharmacy is often a bad deal, according to Interpol and the U.S Food and Drug Administration, announcing a crackdown Thursday on thousands of websites.

The FDA said it has shut down 1,677 sites for selling counterfeit or substandard medication, or for selling drugs without appropriate safeguards. Other sites received regulatory warnings. Officials said they also arrested 58 people and seized more than $41 million worth of illegal medicines.

Several sites had sleek interfaces and names that could easily be confused with legitimate pharmacy retailers. For example, the FDA shuttered Walgreens-Store.com; the well-known drugstore chain's website is actually Walgreens.com.

"It impacts consumers every day," FDA Commissioner Margaret Hamburg told CNN's Dr. Sanjay Gupta. "These products can have none of the active ingredient that people need for the treatment of their disease. They can have too much or too little (of the ingredient); they can have toxic ingredients, and they can prevent patients form getting the actual medications that they badly need to treat their disease."

The most common scams advertised popular drugs such as Viagra, Levitra, Celebrex and Avandaryl. The recent crackdown, labeled Pangea VI, involved the cooperation of more than 100 countries, according to Interpol.

Hamburg acknowledged it's difficult to know the scope of the online pharmacy problem but maintained, "We still do have the safest drug supply in the world."

At some locations, FDA and customs agents use handheld scanners with ultraviolet and infrared radiation to detect suspicious packaging and ingredients. Confirmation of whether a product contains fake ingredients must be done in a laboratory. The scanners are in use at a handful of locations that handle a large volume of imports such as the Los Angeles International Airport.

The FDA also works closely with private pharmaceutical companies, such as Pfizer, that run investigations and turn over evidence to law enforcement. Pfizer declined to say how much it spends on tracking counterfeits or how many people are working on the investigations.

Counterfeits are not isolated to online sales, and it is extremely difficult to know how many deaths and serious illnesses they cause. Unless a patient's adverse reaction is dramatic, the medication is usually not tested, so the vast majority of counterfeit ingredients likely go unnoticed.

But many experts agree the problem is daunting. The National Association of Boards of Pharmacy recently performed an analysis of more than 10,000 websites, and found that 97% did not fully comply with state and federal regulations. It said 88% did not require a valid prescription, and almost half sold medicines lacking FDA approval.

"It's probably a multibillion dollar industry," said Carmen Catizone, executive director of the pharmacy board group.

Catizone said most of these sites are based overseas, with a common scam of site operators presenting themselves as Canadian to lure U.S. customers seeking more affordable medication from a trusted place.

"The fact of the matter is very few, if any, of these sites are actually based in Canada or (are) Canadian," Catizone said. "In fact, they are located in China, India, Pakistan, around the world."

The National Association of Boards of Pharmacy's website features a list of approved online pharmacies as well as those to avoid.

Hamburg said consumers should also watch for a few warning signs. "No. 1, the websites should ask for a valid prescription. If they don't -- beware."

She also said website pharmacies should have a licensed pharmacist available for consultation and should be located in the United States and licensed in the state where the website is registered.

Beyond that, she said, the basics still apply. "If the costs are really low, if it seems too good to be true, it is too good to be true," she said.