What does weight have to do with emergency contraception?

Body weight may reduce the effectiveness of oral emergency contraception (EC). Here’s how pharmacists can help navigate this nuance in your consultations.

A patient walks into a pharmacy and asks for emergency contraception. During the consultation, they disclose that they are breastfeeding, so the pharmacist recommends levonorgestrel.

But what if the patient weighs more than 70 kg?

The effectiveness of oral EC may be reduced by body weight , particularly for levonorgestrel.

Given the average Australian woman weighs 72 kg, weight is an important factor to consider during EC consultations.

Weighing up first-line therapy

Levonorgestrel is widely stocked in Australian pharmacies, with 2024 research revealing 98% of pharmacies have this EC on hand compared with 70% for ulipristal acetate.

This gap is problematic, as for most people, ulipristal is first-line therapy. It’s particularly problematic for people >70kg who wish to use emergency contraception to avoid pregnancy. 

The Australian Pharmaceutical Formulary and Handbook (APF’s) treatment guide for emergency contraception states that for people with a BMI above 26 kg/m² or weight above 70 kg, ulipristal is the preferred first-line option. Above 85 kg or BMI above 30 kg/m², effectiveness is unknown.

Crafting conversations

Part of what makes these thresholds challenging in practice is that weight does not always correlate with what pharmacists might expect.

‘Weight and BMI can be really hard to assess, particularly when people are so fit,’ said Ruth Nona, pharmacist and researcher at James Cook University in Cairns. ‘If it’s levonorgestrel ‘If somebody does CrossFit and has a higher muscle mass, they may be heavier on the scale, but they may not appear so.’

As such, Queensland-based pharmacy intern Lindsay Cameron has developed a consultation approach that sidesteps weight assumptions entirely.

‘I don’t ask for a specific weight. I ask whether they think they may fall into that category,’ she said. ‘I usually say, “I’ll ask a few questions so I can recommend the most appropriate EC for you.” Then I explain, “Ulipristal is generally more effective than other oral EC options, unless it is contraindicated or unsuitable, because it can delay ovulation even when hormone levels are rising. That gives it a longer window to work. In particular, if you think you may weigh over 70 kg, I recommend ulipristal, as the other option may be less effective”.’

Ms Cameron will further tweak her approach, often based on the patient’s body language, if she detects any sensitivity around weight.

‘I might use myself as an example,’ she said. ‘For example, I will say, “I would choose this option because I am over 70 kg and I don’t want an unplanned pregnancy”,’ she said.

Ms Nona also emphasises the importance of ensuring that patients are given all the information they need to make an informed choice. ‘When someone presents to the pharmacy seeking emergency contraception, they have already made an informed decision to prevent pregnancy. It is important that patients understand the differences in effectiveness between the two emergency contraception options, as well as how weight may also influence that effectiveness.’

When oral EC isn’t enough

For patients above 85 kg, the conversation should extend beyond oral EC. The copper intrauterine device (IUD), inserted within 120 hours of unprotected sex, is the most effective option regardless of weight.

However, the main barrier is often access rather than clinical appropriateness. The copper IUD requires insertion by a trained medical professional, and appointments are not always readily available, particularly in regional, rural and remote areas.

The APF is clear that pharmacists should always supply oral EC and refer, not withhold the oral option while waiting for an IUD appointment.

Ms Nona makes this a routine part of conversations with people seeking emergency contraception.

‘When considering BMI, if a patient is thought to weigh over 85 kg, we should always discuss the option of a copper IUD with them, as it is 99% effective,’ Ms Nona said. ‘Another benefit of a copper IUD is that it offers ongoing, long-term contraception.’

‘If they can’t get to the doctor [immediately], we would supply oral EC and refer them to a GP or sexual health clinic,’ she said. 

When ulipristal is contraindicated

While ulipristal is the first-line treatment option, offering protection for a wider weight range, it’s important to understand when it’s contraindicated.

As suggested earlier in the article, ulipristal is generally second line in breastfeeding mothers.

For people >70kg who are breastfeeding, the APF treatment guidelines recommends 3 mg (2 tablets) levonorgestrel as first line treatment. 

However, this is off-label use, and based on international guidance. Its clinical efficacy is unknown. 

‘[As] the effectiveness of double dosing levonorgestrel is not known … we should inform the patient that they should follow up with a doctor.’ Ms Nona said.

The other most common contraindication is drug-drug interactions. The most significant interaction with ulipristal involves progestogens.

‘You cannot use ulipristal and progestogen within 5 days of each other as it reduces the effectiveness of both,’ 

Ms Nona reflects this is something she sees frequently in practice:.

‘That could be because the patient was taking a progestogen-containing oral contraceptive such as Slinda, if they missed a couple of pills or ceased taking it, or if they took levonorgestrel.’

CYP3A4-inducing medicines – including carbamazepine, phenytoin, rifampicin, topiramate and St John’s Wort – used within the previous 4 weeks also affect both oral EC options. In this scenario, the APF recommends a  3 mg dose  of  levonorgestrel – noting that a copper IUD is preferred where feasible, explicitly stating that double-dose ulipristal is not recommended.

For more information on emergency contraception, access the Australian Pharmaceutical Formulary and Handbook.