Morning-after pill ineffective for heavier women
New research indicates the morning-after pill is no use in preventing pregnancies in women weighing more than 80 kilograms and becomes increasingly ineffective in women over 70kg.
Pharmacists will be advised to tell women of the concerns.
Medsafe issued a warning about the effectiveness of the emergency contraceptive levonorgestrel, available as Postinor-1, at the end of February.
It said recent publications had indicated levonorgestrel may not be effective for women who weighed more than 70kg.
Levonorgestrel is used as an oral emergency contraceptive within 72 hours of unprotected sex.
"Irrespective of weight, levonorgestrel may not prevent pregnancy in every case. The sooner you take emergency contraception, the more likely it is that it will work," the warning said.
Medsafe said it was continuing to review concerns about the emergency contraceptive in conjunction with the Therapeutic Goods Administration in Australia. Further information would be provided when the review was complete.
Pharmacy Guild professional services and support manager Linda Caddick told Radio New Zealand that the guild was aware of Medsafe's warning.
She could not find any reference to the weight concerns in written information given to women buying emergency contraceptives over the counter in this country.
Usually the guild would wait until it received official notification from Medsafe about concerns before making a statement.
"In light of it [emergency contraception] being so highlighted in the media, we would certainly be giving pharmacists advice now to tell women that if they are over 70kg it will be less effective, and international studies would show that over 80kg [it is] not effective at all," she said.
The European Medicines Agency said in January that it had started a review of emergency contraceptives containing the hormone levonorgestrel to assess whether increased weight and body mass index reduced their effectiveness.
That followed a procedure finalised last November to add information about weight concerns to the packaging of HRA Pharma's Norlevo emergency contraceptive.
The changed packaging information said: "In clinical trials, contraceptive efficacy was reduced in women weighing 75kg or more, and levonorgestrel was not effective in women who weighed more than 80kg."
The company began investigating the need to change Norlevo's label after research at the University of Edinburgh, published in 2011, showed emergency contraceptive pills that used levonorgestrel were prone to fail in women with a higher body mass index.
According to Bloomberg, based on World Health Organisation data, the average weight of New Zealand women is 74.6kg.
Family Planning medical director Christine Roke said latest research indicated the effectiveness of the emergency contraceptive started to decline once users weighed more than 70kg.
Family Planning had been advising its clients about the issue for a couple of years after concerns were raised at a United States conference, even though the research had not been confirmed then.
While most women chose to take the pill because it was easy, Family Planning had been advising women that a copper IUD would be more effective, particularly for heavier women, as emergency contraception, Roke told Radio New Zealand.
Family Planning strongly recommended women weighing more than 70kg consider an IUD.
Roke and Caddick said the women they dealt with who wanted emergency contraception tended to be lighter than average and to be young.
"If somebody is more mature and in a relationship they are less likely to seek emergency contraception," Caddick said.
After a quick calculation, Caddick suggested emergency contraception was provided 130,000 times a year over the counter at pharmacies, while Radio New Zealand said 16,000 prescriptions for the emergency contraceptive pill were given out by GPs each year.