The Food and Drug Administration is warning consumers to avoid certain vials of insulin made by Novo Nordisk that were stolen earlier this year and may not be safe for use.

The agency said Wednesday that the vials may not have been stored properly and could be dangerous to consumers. The FDA has received three reports of patients who suffered unsafe blood sugar levels after using the products.

The FDA issued an alert in June after three lots of Novo Nordisk's Levemir insulin were stolen during shipment through Johnston County, N.C. Regulators say only about 2 percent of the 129,000 vials have been returned.

Regulators advised patients not to use Levemir insulin from lots: XZF0036, XZF0037, XZF0038.

"Replace it with a vial of Levemir from another lot," the agency said in a statement. "If you must switch to another brand of insulin for any reason, first contact your health care provider because another insulin product may require adjustments in dosing."

The FDA says it continues to investigate the theft.

A spokesman from Novo Nordisk's U.S. headquarters in Princeton, N.J., said the company is cooperating with the FDA. Novo Nordisk is based in Denmark.

People with type 2 diabetes have trouble breaking down carbohydrates because their bodies have become resistant to insulin, a protein which occurs naturally in the body and is critical to digesting sugars. Over time, diabetics are at higher risk for heart attacks, kidney problems, blindness and other serious complications.