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If I am using enzyme inducers (such as Dilantin, the antibiotics rifampicin or griseofulvin, or St. John's Wort), will it make emergency contraceptive pills less effective? Are the instructions for using the pills different?
Medications and herbal supplements that may make regular birth control pills less effective may also reduce the effectiveness of emergency contraceptive pills. This applies to both progestin-only pills, (like Plan B One-Step, Next Choice One Dose, Next Choice and Levonorgestrel Tablets), and ulipristal acetate (ella). So if you are using an enzyme inducer (such as Dilantin, the antibiotics rifampicin or griseofulvin, or St. John’s Wort), it probably makes sense to increase the dose of emergency contraceptive pills (also known as "morning after pills" or "day after pills").
For these enzyme inducers, there are no official recommendations in the United States for what to do in this situation, but we can give guidance for users of progestin-only pills based on what medical experts suggest in the United Kingdom, where emergency contraceptive pills have been available longer. The Faculty of Family Planning and Reproductive Health Care Clinical Effectiveness Unit there advises patients taking liver enzyme inducers who are also using progestin-only emergency contraceptives (like Plan B One-Step, Next Choice One Dose, Next Choice and Levonorgestrel Tablets) to take 3 mg levonorgestrel. This would be two Plan B One-Step or Next Choice One Dose tablets or four Next Choice or Levonorgestrel Tablets tablets, at one time. Read more here.
If you are using “combined” birth control pills for emergency contraception (which contain progestin and estrogen, such as regular birth control pills used as EC), you should double the first dose of hormones you take (for more information about the standard doses and instructions for emergency contraceptive pills, click here). Alternatively, some health care providers suggest that you increase the number of pills in each dose by 50%, meaning that you take one and half doses each time (for more information on specific dosages birth control pills used for emergency contraception, click here).
As of yet, there are no guidelines for how to adjust ulipristal acetate (ella) intake if you're using enzyme inducers.
In general, if you increase your dose of emergency contraceptive pills, you are more likely to feel sick to your stomach, which is already one of the common side effects. To avoid that, you can try taking 25-50 mg of the anti-nausea medication meclizine (sold as a generic drug or under the brand names Dramamine II or Bonine in the United States) one hour before taking the first dose of emergency contraception.
A thorough and up-to-date academic review of the medical and social science literature on emergency contraception is available; click here for the PDF.