Morning-after pill mechanism: An explanation of its functional aspects
Morning-after pills, such as Plan B and ella, are emergency contraceptives designed to prevent pregnancy following unprotected sex. However, it's essential to understand that these pills are not the same as abortion pills, which are taken after a pregnancy has begun.
When it comes to the effectiveness of morning-after pills, body mass index (BMI) can play a role. Doctors still suggest that anyone worried about pregnancy after unprotected sex should take the morning-after pill, regardless of their BMI.
Progestin pills, like Plan B, can reduce the chance of pregnancy by 81% to 90% when taken within three days of unprotected sex. On the other hand, anti-progestin pills, like ella, can lower the chance of pregnancy by about 85% if taken within five days.
However, studies have generally shown that the pills' effectiveness dips in people with BMIs over 30, which is in the "obese" range, though select studies show dips at BMIs of 25, in the "overweight" range. People with BMIs over 30 have about four times higher chance of pregnancy after taking these pills compared to those with lower BMI.
In contrast, anti-progestin pills have been shown to be more effective than progestin pills for people with BMIs of 35 or higher.
The exact reason for this BMI-dependent difference is unclear, though hypotheses include altered drug concentrations due to body fat or hormonal variations affecting ovulation. However, these are not confirmed.
An important alternative to morning-after pills, unaffected by BMI, is the copper intrauterine device (IUD). This method is highly effective (>99.9%) for emergency contraception when inserted within five days post-unprotected sex and is not influenced by body weight or BMI.
In summary, the efficacy of morning-after pills varies with BMI. Progestin-based emergency contraceptives become significantly less effective in individuals with a BMI over 30, while anti-progestin pills retain better efficacy but may begin to lose effectiveness at BMIs above 35. The copper IUD, on the other hand, remains highly effective regardless of BMI.
Therefore, people with higher BMI using morning-after pills should be aware of potentially reduced efficacy and consider alternatives like the copper IUD for emergency contraception.
| Morning-After Pill Type | BMI Impact | Notes | |-----------------------------|-----------------------------------------------|---------------------------------------------------| | Progestin-based (Levonorgestrel) | Effectiveness drops significantly > BMI 30, possibly from BMI 25 | Higher pregnancy risk despite pill use; no proven benefit from extra dose | | Anti-progestin (Ulipristal acetate - ella) | Slightly more effective at higher BMIs; decline may start > BMI 35 | More reliable in high BMI but still some reduction | | Copper IUD | No impact from BMI | Most effective emergency contraceptive method |
[1] Study 1 [2] Study 2 [3] Study 3
- It's worth noting that the effectiveness of progestin-based morning-after pills, such as Plan B, could significantly decrease for individuals with a BMI over 30, while anti-progestin pills, like ella, may still be more effective but begin to lose effectiveness at BMIs above 35.
- For individuals with higher BMIs, the copper intrauterine device (IUD) is an alternative to morning-after pills, as it remains highly effective for emergency contraception, regardless of BMI.
