01Purpose and principlesWhat the treatment does and how it fits into care.
Hepatic enzyme induction, particularly of CYP3A4 and related pathways, increases clearance of ethinylestradiol and progestogens. Serum hormone exposure can fall below that required to suppress ovulation or maintain cervical-mucus effects. Breakthrough bleeding may signal reduced exposure but its absence does not prove protection. The interaction begins after the inducing medicine is started and persists for 28 days after it stops while enzyme expression returns towards baseline.
Take a complete medicine history before prescribing contraception and whenever another clinician starts treatment. Ask specifically about antiseizure medicines, rifamycins, antiretrovirals, herbal products and over-the-counter preparations. Common important inducers include carbamazepine, phenytoin, phenobarbital, primidone, oxcarbazepine, rifampicin, rifabutin and St John’s wort. CoSRH advises treating topiramate as a potential inducer regardless of dose because exposure varies and the medicine is teratogenic.
Methods vulnerable to induction are all CHC routes, not only the pill: combined oral pills, patch and vaginal ring; traditional, desogestrel and drospirenone POPs; and the etonogestrel implant. Oral levonorgestrel and ulipristal emergency contraception are also affected. Increasing routine POP or implant dose is not validated. A patch or ring does not escape the interaction because systemic steroid metabolism still determines exposure.
The unaffected methods are the copper IUD, LNG-IUD and DMPA. Copper provides hormone-free contraception and is the emergency method of choice. LNG-IUD effectiveness is mainly local and is not reduced by induction. DMPA achieves systemic levels sufficient to remain effective, using the standard 13-week dose schedule. Select among them with medical eligibility, bleeding, procedure, bone-health, fertility-return and patient-preference considerations.
Duration shapes management. For long-term or repeated induction, recommend an unaffected method rather than serial barrier workarounds. For a short course under two months in an established CHC user, carefully used condoms throughout treatment and for 28 days afterwards may be acceptable in selected cases, or specialist guidance may use a higher total ethinylestradiol dose with continuous or tricycling regimens. Do not use this workaround with the potent inducers rifampicin or rifabutin, and do not double patches or rings.
Emergency contraception requires modification. Offer a copper IUD first because enzyme induction does not affect it and it is the most effective EC. If declined or unsuitable, ulipristal is not recommended during induction or within 28 days of stopping. Levonorgestrel 3 mg may be offered instead of 1.5 mg, while clearly explaining that evidence for effectiveness is limited. Arrange an unaffected ongoing method and pregnancy testing 21 days after the latest unprotected intercourse.
Distinguish enzyme induction from other interactions. Lamotrigine does not induce contraceptive metabolism. Instead, oestrogen in CHC can substantially lower lamotrigine concentrations, risking seizure recurrence, while the hormone-free interval can raise concentrations and adverse effects. Specialist collaboration is needed before starting or stopping CHC; progestogen-only and non-hormonal alternatives may be simpler. Some antiretroviral regimens inhibit rather than induce enzymes and require drug-specific interaction checking rather than class assumptions.
The prescriber of either medicine shares responsibility for prevention. Do not tell a patient to stop essential antiseizure, tuberculosis or HIV treatment. Contact the relevant specialist or pharmacy service, document the exact inducer, start and stop dates, contraceptive method and EC plan, and communicate changes across services. Where the medicine is teratogenic, ensure effective contraception is established before treatment whenever clinically possible and provide pregnancy advice without blame if exposure occurs.
Key points
- Enzyme inducers increase hepatic metabolism of contraceptive steroids and can reduce effectiveness of combined pills, patch and ring, progestogen-only pills, etonogestrel implant and oral emergency contraception.
- Copper IUD, levonorgestrel IUD and depot medroxyprogesterone acetate are not reduced by hepatic enzyme induction and are the preferred reliable options when acceptable.
- Important inducers include carbamazepine, phenytoin, phenobarbital, primidone, oxcarbazepine, rifampicin, rifabutin and St John’s wort; treat topiramate as a potential inducer regardless of dose under current CoSRH caution.
- Interaction precautions continue throughout the inducer course and for 28 days after it stops because enzyme activity does not normalise immediately.
- For short courses under two months, temporary condoms during treatment and for 28 days after may be considered with some established methods, but potent rifampicin or rifabutin needs an unaffected strategy.
- For long-term induction, switch to copper IUD, LNG-IUD or DMPA rather than relying on repeated emergency contraception or unproven dose increases.
- After unprotected intercourse with current or recent induction, offer copper-IUD EC; if unsuitable or declined, ulipristal is not recommended and levonorgestrel 3 mg may be considered with uncertain effectiveness.
- Lamotrigine is not an enzyme inducer: CHC lowers lamotrigine concentrations and the hormone-free interval raises them, so specialist management is needed for seizure control and toxicity.
02Indications, selection and cautionsWho may benefit, who needs urgent treatment and important alternatives.
Carbamazepine, phenytoin, phenobarbital, primidone, oxcarbazepine, rifampicin, rifabutin and St John’s wort are recurring high-yield interaction triggers.
Current CoSRH guidance treats topiramate as a potential enzyme inducer regardless of dose because pharmacokinetics vary and fetal exposure carries major harm.
New bleeding during induction can reflect reduced steroid exposure, but normal bleeding does not guarantee that contraceptive concentrations remain effective.
Contraceptive precautions continue for 28 days after the inducer is stopped; removing precautions on the final tablet day is premature.
CHC reduces lamotrigine during active hormones and concentrations rebound in the break, risking both seizures and dose-related adverse effects.
03Assessment before treatmentTests and checks that guide safe selection.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Exact medicine reconciliationFirst step - Why
- Identify the inducer, dose, indication, duration and start or stop date before contraceptive failure occurs.
- Interpretation and limitations
- Check prescription records, herbal products and specialist plans. Brand names can obscure ingredients, and topiramate is managed cautiously at any dose.
- 02
Contraception chronology - Why
- Determine whether induction overlapped a vulnerable method and recent intercourse.
- Interpretation and limitations
- Record exact pill, patch, ring, implant or injection use and every intercourse date through 28 days after induction; assess EC and pregnancy testing from this timeline.
- 03
Pregnancy testing - Why
- Detect pregnancy after compromised contraception or before switching to a method with procedural implications.
- Interpretation and limitations
- A baseline negative result cannot exclude recent conception. Repeat 21 days after the latest at-risk intercourse and assess pain or bleeding urgently.
- 04
Drug-specific interaction check - Why
- Separate induction from inhibition, absorption effects and reciprocal contraceptive effects on another medicine.
- Interpretation and limitations
- Use current CoSRH, BNF, product and specialist resources; do not infer the interaction from therapeutic class alone, especially with antiretrovirals or lamotrigine.
- 05
Selective lamotrigine level and clinical review - Why
- Support safe seizure or mood management when CHC is started, changed or stopped.
- Interpretation and limitations
- Clinical status is primary; levels may document a personal baseline and fluctuation. Dose changes require the prescribing specialist because hormone withdrawal can increase exposure.
04Treatment approachPreparation, options, escalation and aftercare.
01First-line unaffected optionsChoose IUD or DMPAFirst stepFirst lineA patient will use an enzyme-inducing medicine for a prolonged or uncertain duration.+
- 1Confirm the exact inducer, teratogenic risk, expected duration, current contraception, recent intercourse and medical eligibility.
- 2Offer copper IUD, LNG-IUD or standard-schedule DMPA as reliable methods unaffected by induction, comparing bleeding, procedure, bone and fertility-return effects.
- 3Bridge safely until the new method is effective and continue inducer-related precautions for 28 days after the final dose if a vulnerable method is resumed.
02Short courseProtect through enzyme washoutA non-rifamycin inducer is prescribed for less than two months to a patient using vulnerable hormonal contraception.+
- 1Discuss switching to an unaffected method; if the established method continues, use condoms consistently during the entire course and for 28 days afterwards under current guidance.
- 2For selected combined-pill users only, obtain specialist advice on at least 50 micrograms total ethinylestradiol with continuous or tricycling use and a shortened four-day break rather than improvising doses.
- 3Do not double patches or rings and do not use workaround regimens with rifampicin or rifabutin; move to an unaffected method instead.
03Emergency exposureUse copper-IUD EC firstUnprotected intercourse occurred while induction could compromise the method.+
- 1Offer a copper IUD within the valid emergency window and assess pregnancy, infection, anatomy and consent promptly.
- 2If it is declined or unsuitable, avoid ulipristal and consider levonorgestrel 3 mg with transparent counselling that effectiveness evidence is limited.
- 3Start an unaffected ongoing method, provide backup and perform a pregnancy test 21 days after the latest at-risk intercourse.
04LamotrigineManage the reciprocal interactionA person using lamotrigine requests CHC or plans to stop an existing combined method.+
- 1Clarify indication, seizure control, lamotrigine dose and hormone regimen and involve the specialist prescriber before any change.
- 2Explain that active CHC can lower lamotrigine and the hormone-free interval or CHC cessation can raise it, causing alternating loss of control and toxicity.
- 3Prefer a suitable non-CHC method when acceptable or coordinate dosing, regimen and monitoring explicitly across contraceptive and neurology or psychiatry care.
05Regimens, contraindications and interactionsTreatment details and the circumstances that modify them.
Depot medroxyprogesterone acetate during enzyme induction
Administer 150 mg intramuscularly or 104 mg subcutaneously every 13 weeks using the standard schedule; no dose increase is required for hepatic enzyme induction.Discuss bleeding, weight, bone density, delayed fertility return and inability to remove a dose; confirm timely repeat administration and individual UKMEC suitability.
Levonorgestrel emergency contraception with enzyme induction
If copper-IUD emergency contraception is declined or unsuitable, give levonorgestrel 3 mg orally once during current induction or within 28 days of stopping, repeating if vomiting occurs within three hours.Effectiveness of the double dose is uncertain; do not substitute ulipristal, arrange ongoing unaffected contraception and test for pregnancy 21 days after the latest risk.
06Complications, monitoring and follow-upAdverse effects, response and longer-term review.
- Maintain a shared record of the inducer’s exact start and stop dates and extend precautions for the full 28-day washout period.
- Check that a planned IUD or DMPA method was supplied and effective before a vulnerable pill, patch, ring or implant is considered covered.
- Arrange pregnancy testing 21 days after the latest intercourse affected by induction and escalate pain, bleeding or a positive result for location assessment.
- Monitor seizure control and lamotrigine adverse effects around CHC initiation, hormone-free intervals and cessation under specialist direction.
- Revisit the interaction whenever medicines are reconciled, because short antibiotic or herbal histories may be omitted unless asked by name.
07Special situationsVariants, exceptions and circumstances that change the usual approach.
Non-oral CHC is still affected
Patch and ring avoid gastrointestinal absorption but their circulating steroid hormones are still cleared more rapidly by induced hepatic enzymes.
The interaction outlasts treatment
Induced enzymes persist after the final medicine dose, creating a 28-day contraceptive tail that must be included in advice.
Local or high-dose methods endure
LNG-IUD action is predominantly uterine and DMPA exposure remains sufficient, while copper contains no steroid to metabolise.
Topiramate gets a cautious rule
Rather than relying on a dose threshold, current UK guidance treats it as potentially inducing because exposure varies and pregnancy carries teratogenic risk.
Lamotrigine runs the other way
The major concern is CHC changing lamotrigine, not lamotrigine inducing contraception, which is why routine inducer advice alone is unsafe.
08Common pitfallsFrequent interpretation and management errors.
- 01
Checking interactions only for oral pills and missing that patch, ring and implant effectiveness can also fall.
- 02
Stopping additional precautions on the last inducer dose instead of continuing for 28 days.
- 03
Recommending standard-dose oral emergency contraception or ulipristal during recent enzyme induction.
- 04
Doubling a POP, implant, patch or ring without evidence rather than offering an unaffected method.
- 05
Calling lamotrigine an inducer and overlooking CHC-driven fluctuation in lamotrigine levels.
- 06
Stopping essential antiseizure or antituberculous treatment instead of coordinating a safer contraceptive method.