The number of 'rogue' wholesale distributors selling fake or unapproved prescription drugs is growing, so doctors need to be vigilant when purchasing medicines, the U.S. Food and Drug Administration warned Tuesday.

In a statement, the agency said the problem is so widespread that it has launched a program to educate doctors and other health care providers and administrators about proper drug purchasing procedures. The program aims to protect patients from taking potentially harmful counterfeit drugs.

The "Know Your Source" program urges health care professionals to only buy prescription drugs from wholesale drug distributors licensed in their states. Doing so will reduce the risk of giving unsafe or ineffective drugs to patients.

The FDA cautioned physicians to avoid offers that sound too good to be true, and to question aggressive marketing tactics and major discounts on prescription drugs suggesting that the medicines may be stolen, fake, substandard or unapproved.

The agency urged doctors to verify that they are only receiving FDA-approved products. Unapproved drugs may contain unknown or harmful ingredients, or may not have been made, transported or stored under proper conditions, the agency said.