A chemical used in manufacturing rubber was found in a Cubist Pharmaceuticals Inc. antibiotic stored in ReadyMED pumps made by Cardinal Health Inc., the U.S. Food and Drug Administration said on Wednesday.

The chemical called mercaptobenzothiazole or MBT can irritate the skin and its chronic administration has been associated with an increase risk of certain tumors.

Eileen McIntyre, spokeswoman for Cubist said that the impurity was found during routine testing but that the contamination wasn't linked to Cubicin (the antibiotic) but rather to Cardinal's drug pump.

Cubist Pharmaceutics is advising healthcare professionals to avoid the use of ReadyMED pumps with Cubicin.

Shares of Cubist were off 0.58 cents or 2.88 percent at $19.53.