Diane-35, a drug used to treat severe, stubborn acne, should never be prescribed as a birth control pill, Health Canada says.

The controversial medication also should never be given to women who have a history of blood clots, the department said in an advisory issued Thursday.

These and other warnings are included on updated product information that is now being packaged with the drug.

The department issued the warning after being tipped off that some doctors were prescribing it solely as a contraceptive.

When Health Canada approved Diane-35 for sale in 1998, it said it was only to be used as a last resort for acne that didn't respond to antibiotics.

The department included a rare restriction for doctors, warning that the drug should never be prescribed solely as a contraceptive.

While Diane-35 does prevent pregnancy, a U.S. study also found it increases the risk of blood clots by up to four times more than standard birth control pills.

In a 2003 investigation, the CBC program Disclosure found that more than 800,000 prescriptions for the drug were written for Canadian women in 2002 and the majority were taking it for birth control.

In Thursday's warning, Health Canada asked patients to tell their doctors if they have had blood clots in the legs, lungs, eyes or elsewhere, or a stroke, heart attack or chest pain.