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Women have been warned that widely-used morning-after pills might not prevent pregnancy if they weigh more than 70kg.
Thousands of women in New Zealand take the pill, but the Ministry of Health has issued a precautionary alert after overseas research raised concerns over the emergency contraception's effectiveness.
The ministry is reviewing the research before deciding on any regulatory changes.
These could involve warnings on information sheets or packaging.
The average weight of women in New Zealand is over 72kg. About 16,000 prescriptions were given out last year for Postinor-1, the sole state-funded emergency contraceptive pills - but it and two other brands, Next Choice and Next Choice Arrow, can also be bought from pharmacists.
The ministry notes that irrespective of a woman's weight, the pill - which is intended to be taken within 72 hours of unprotected sexual intercourse - may not always prevent pregnancy. The sooner it is taken, the more likely it is to work.
Family Planning says the pregnancy rate after emergency contraceptive pill (ECP) use among women who are technically obese - those with a body mass index over 30 - "is the same as for women who don't take an ECP at all".
It says the research - which has prompted reviews in several countries - found a pregnancy rate among obese women who used levonorgestrel (the active ingredient in Postinor-1) or another emergency pill was three times that of women whose BMI was under 30.
"Women who come to a Family Planning clinic needing emergency contraception are given the option of a post-coital IUD [the Multiload copper intra-uterine device] instead of the emergency contraceptive pill because it is more effective," the organisation says in its Forum newsletter.
"... when used as emergency contraception [it] is more than 99 per cent effective no matter how much a woman weighs. Also, if the woman chooses to keep the IUD in place, it provides up to five years of highly effective contraception."
Family Planning's medical director, Dr Christine Roke told the Herald: "We've been encouraging women, particularly the heavier ones, to consider having the IUD, but most feel more comfortable with taking the pill."
She said it was not known why the emergency contraceptive pill was less effective in heavier women.
It was thought that it took longer for hormones in heavier women to reach the required level.
"While that doesn't matter when you're taking a contraceptive pill each day, it matters for emergency contraception because you want to get the levels up as soon as possible."
Physician and clinical pharmacology fellow Dr Jenny Robinson wrote on Australian website The Conversation that although the concerns raised by the research were serious, the small number of overweight and obese women in the studies and the low number of pregnancies meant "we should be cautious in applying the results too broadly".
In the research, 1731 women took levonorgestrel pills and 38 became pregnant, while 1714 women took ulipristal acetate and 22 became pregnant.
Obese women were 13.6 per cent of the study group, and overweight women 21.6 per cent.
Morning-after pill warning
• Researchers discover widely-used emergency contraceptive has higher chance of failure if woman weighs more than 70kg
• Health ministry issues alert, and Family Planning says it offers another option 'because it is more effective'