Perrigo has repeatedly failed to correct multiple manufacturing violations at its Allegan, Mich., facility that have resulted in drug mix-ups and contamination, according to U.S. health regulators.

Shares of the drug manufacturer and distributor fell 6% to $53.61 in early trading on the Nasdaq after the U.S. Food and Drug Administration (FDA) released its warning letter to the company citing the problems.

In the letter, dated April 29, FDA said Perrigo repeatedly did not properly inspect products despite a program implemented in 2005 to try to prevent such mishaps. For example, FDA said the company failed to reject one lot of ibuprofen tablets contaminated with metal shavings and did not thoroughly investigate when different orange and brown versions of the painkiller were mixed up. It also did not properly check the labels for various milk of magnesia products, the agency said.

"Your firm continues to receive complaints regarding this issue, and despite past assurances that previous enhancements would control this problem, deviations continue," the FDA said. Perrigo "failed to implement sustainable corrective actions to prevent mix-ups as well as other continuing problems," the agency wrote. "As a result, we have concerns regarding the failure of your firm ... to act proactively to ensure compliance with" FDA manufacturing regulations. Representatives for Perrigo could not be immediately reached for comment. FDA posted the letter on its website.