Emergency contraceptive pill packages will soon carry warning messages about reduced effectiveness for heavier women, Health Canada said.

Health Canada said in a statement on Wednesday that it has asked companies to add new warnings to packages advising women that the contraceptive, also known as the morning-after pill, is less effective in women who weigh between 165 and 176 pounds, Health Canada said in a statement Tuesday. The pill is ineffective in women who weigh more than 176 pounds.

Emergency contraceptive pills are taken by women up to 72 hours after unprotected sex or a contraceptive accident, such as a condom breaking, to prevent pregnancy.

The pills contain a higher dose of the same hormone found in daily birth control pills, and work by preventing ovulation or fertilization. They have no effect if a woman is already pregnant, the statement said.

Currently, the products do not feature warning labels about weight-based effectiveness, but Health Canada said it has sent letters to Canadian drug manufacturers requesting they update their packaging. Contraceptive packages without the warnings will remain on pharmacy shelves during this transition time, Health Canada said.

Health Canada's statement came two months after the federal drug regulator began evaluating new data on the pill's efficacy, provoked when a French drug manufacturer announced its pill doesn't work on larger women.

France's Laboratoire HRA Pharma manufactures Norlevo, one of the emergency contraceptive pills available in Canada. In January, the European Medicines Agency said in a statement that it was in the process of reviewing the contraceptive. Health Canada began its review at the same time.

HRA Pharma is in the process of changing the Norlevo packaging to reflect the recent research, but the other three "morning after" pills available in Canada do not have warnings. The other pills available to Canadian women without prescription are Next Choice, Option 2 and Plan B.

Health Canada recommends that women who weigh more than 165 pounds speak to a doctor or pharmacist about alternative methods of emergency contraception.