Plan B with the birth control pill
Many women of reproductive age take a combined or progestogen-only oral contraceptive while also using a chronic medication such as Plan B (Levonorgestrel). The combination is generally fine at 1.5mg, but a small number of medications can reduce contraceptive efficacy meaningfully and need either a backup method or a switch.
How Plan B can affect contraceptive efficacy
Combined and progestogen-only contraceptives are metabolised through CYP3A4. Strong CYP3A4 inducers (some antiepileptics, rifampicin, St John's Wort) lower contraceptive plasma levels and reduce efficacy. Whether Levonorgestrel acts on CYP3A4 determines whether Plan B affects contraception. Most agents in Hormones and Birth Control have no clinically meaningful effect on the pill at 1.5mg.
Practical guidance
According to the prescribing information for Levonorgestrel, women on hormonal contraception should review Plan B with the prescribing pharmacist or doctor. Where an interaction is documented, additional barrier contraception or switching to a non-oral method (IUD, implant) for the duration of Plan B therapy is the standard mitigation.
Frequently asked questions
Will Plan B make my pill less effective? ▾
Most Hormones and Birth Control medications at 1.5mg do not affect oral contraceptive efficacy. The exceptions are CYP3A4-inducing drugs and a small number of others. The prescribing information for Levonorgestrel states whether the interaction is meaningful.
Do I need a backup contraceptive on Plan B? ▾
Backup contraception is needed only when there is a documented interaction between Plan B and the contraceptive method. For most users at 1.5mg, no backup is required. The pharmacist confirms whether Levonorgestrel interacts with hormonal contraception.
More on Plan B
- With alcoholPlan B and alcohol — is it safe to drink?
- With foodShould Plan B be taken with food?
- Side effectsPlan B side effects: common, rare and warning signs
- Dosage guidePlan B dosage guide: how much to take and when
- OnsetHow fast does Plan B start working?
- DurationHow long does Plan B last?
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.