Hello,
Thanks for writing to DoctorSpring.com with your health concern.
Penicillin is an enzyme inducer, and it will lower the efficacy of the emergency contraceptive Plan B. There are no clear cut guidelines for users who take the two medications together.
However, a general recommendation is to take double the dose of Plan B. So please double the dose accordingly.
You might definitely feel nausea and some amount of sickness due to the doubled hormone dosage. 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.
All the best.
Please feel free to ask for further clarifications.