The US Food and Drug Administration launched a national campaign Friday alerting consumers to the risks of phony web pharmacies.

The warning comes as nearly one in four people who shop online say they have bought prescription drugs on the Internet, according to the FDA.

"Buying medicines from rogue online pharmacies can be risky because they may sell fake, expired, contaminated, not approved by FDA, or otherwise unsafe products that are dangerous to patients," FDA Commissioner Margaret Hamburg said in a statement.

"Fraudulent and illegal online pharmacies often offer deeply discounted products. If the low prices seem too good to be true, they probably are."

False online drug stores use sophisticated marketing techniques or fake Web storefronts to make themselves appear legitimate, the FDA said.

Drugs bought from such sources may contain the wrong or even harmful ingredients, putting patients' health at risk, it added.

To avoid being scammed, members of the public who choose to buy medication online should pick pharmacies that, among other things, require a valid prescription, are located in the United States and have a licensed pharmacist available for consultation, the FDA recommended.

[Image via Agence France-Presse]