(CNN) The US Food and Drug Administration has issued a warning letter to Dollar Tree for receiving over-the-counter drugs produced by foreign manufacturers that have been found to be adulterated, including acne treatment pads and Assured brand drugs.

The letter -- sent earlier this month to Greenbrier International Inc., which does business as Dollar Tree -- describes Dollar Tree's receipt of adulterated drugs from manufacturers and suppliers that had received separate FDA warning letters last year and were placed on " import alert ."

The letter outlines "multiple violations" of manufacturing practices at those contract manufacturers used to produce Dollar Tree's Assured Brand over-the-counter drugs as well as other drug products sold at Dollar Tree and Family Dollar stores.

Now, in its warning letter to Dollar Tree, the FDA is requesting that the company implement a system to ensure that it does not import adulterated drugs.

"Americans expect and deserve drugs that are safe, effective and that meet our standards for quality. The importation and distribution of drugs and other products from manufacturers that violate federal law is unacceptable," Donald D. Ashley, director of the Office of Compliance in the FDA's Center for Drug Evaluation and Research, said in a news release on Thursday.

Read More