Answers to Frequently Asked Questions About...
Effectiveness
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. 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
you guidance 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) to take 2.25 mg levonorgestrel, or three Plan B tablets, at
one time.
If you are using “combined”
birth control pills for emergency contraception (which contain
progestin and estrogen), 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
for Plan B and other birth control pills used for emergency contraception,
click here).
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.