The U.S. Food and Drug Administration warned doctors about a manufacturing change in heparin that's expected to decrease the potency of the drug.

The change, adopted by the United States Pharmacopeia, or USP, a nonprofit standards-setting organization, includes a revised reference standard for the drug's unit dose. The changes also require companies to test for a contaminant that was found in batches of heparin last year that were manufactured with ingredients from China, including heparin sold by Baxter International Inc. (BAX).

The change went into effect Thursday, but the FDA has asked four companies who market heparin in the U.S. to hold off shipping new heparin products until Oct. 8 to allow hospital pharmacies time to adjust dosing practices. Heparin is used in many surgery patients.

The FDA said the revised USP reference standard and unit definition for heparin is about 10% less potent than the former USP unit.

"Although the FDA-approved labeling for heparin has not changed, including the recommended doses, it is essential that health care professionals be aware of the potential difference in potency between the old and new vials of heparin when administering the drug," the FDA said in a statement.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com