The U.S. Food and Drug Administration found that a drug ingredient manufactured at a GlaxoSmithKline Plc plant in Ireland was contaminated and said the company did not take sufficient action to resolve the problems.
GSK said the ingredient was paroxetine, used to make its antidepressant drugs Paxil and Seroxat.
The company said it had proposed a recall of certain batches of the drugs from wholesalers but there was no risk of harm to patients taking these drugs.
In a warning letter dated March 18, the FDA said GSK did not fully investigate a list of objectionable conditions the regulator sent after its inspection of the plant at Cork in October.
Some batches of a drug or drugs using the contaminated ingredient were later shipped, the FDA said, and GSK did not notify its customers about the lapse.
The FDA said its investigator found that a certain drug ingredient was contaminated with material from the plant's pharmaceutical waste tank.
The FDA said it might withhold approval of any new drug applications that list GSK as the manufacturer of their drug ingredients until GSK corrected the issues.
The regulator said it also might refuse the import of drugs manufactured at the Cork facility into the United States.
The Cork plant is the company's only drug ingredient manufacturing facility in Ireland.
"Cork is the main site for manufacturing the API (paroxetine), we do buy in some from a third party, but Cork is the main site where we produce it," a company spokesman told Reuters.
Over-the-counter medicines and oral care products are manufactured at a plant in Dungarvan, while consumer products are made at Sligo, both in Ireland.
GSK recorded sales of 285 million pounds ($474.11 million) for Paxil and Seroxat in 2013, the spokesman said.
The investigator also raised concerns about the suitability of the equipment used to manufacture the ingredients.
The news comes just days after GSK said it was recalling all supplies of its over-the-counter weight-loss drug in the United States and Puerto Rico, after concerns that bottles had been tampered with.
The FDA has stepped up its efforts to ensure drug safety in recent months, banning drugs and drug ingredients imported from Indian manufacturers over quality concerns.
Ranbaxy Laboratories Ltd has been banned from exporting drugs from its Indian plants to the United States.
One of Sun Pharmaceutical Industries Ltd's plants and some of Wockhardt Ltd's plants have also been barred from exporting to the United States.