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Purpose and principlesIndications, selection and cautionsAssessment before treatmentTreatment approachRegimens, contraindications and interactionsComplications, monitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Contraceptive interactions with enzyme-inducing medicines

Obstetrics and gynaecology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

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An interaction can create immediate pregnancy risk

Unprotected intercourse during enzyme induction or within 28 days after it ends can represent contraceptive failure for CHC, POP, implant and oral emergency contraception.

Action: Reconstruct medicine, contraception and intercourse dates today, offer copper-IUD emergency contraception first, consider double-dose levonorgestrel only when needed, and establish an unaffected ongoing method.

Purpose and principles

Prevent contraceptive failure by recognising hepatic enzyme induction, selecting unaffected methods, managing short and long courses, modifying emergency contraception and separating induction from other important interactions.

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.

Indications, selection and cautions

Classic inducer list

Carbamazepine, phenytoin, phenobarbital, primidone, oxcarbazepine, rifampicin, rifabutin and St John’s wort are recurring high-yield interaction triggers.

Topiramate caution

Current CoSRH guidance treats topiramate as a potential enzyme inducer regardless of dose because pharmacokinetics vary and fetal exposure carries major harm.

Breakthrough bleeding signal

New bleeding during induction can reflect reduced steroid exposure, but normal bleeding does not guarantee that contraceptive concentrations remain effective.

Persistent post-course effect

Contraceptive precautions continue for 28 days after the inducer is stopped; removing precautions on the final tablet day is premature.

Lamotrigine reverse interaction

CHC reduces lamotrigine during active hormones and concentrations rebound in the break, risking both seizures and dose-related adverse effects.

Red flags requiring action

  • A patient taking a teratogenic medicine such as topiramate or some antiseizure drugs without reliable unaffected contraception needs urgent specialist coordination before exposure or pregnancy occurs.
  • Seizure recurrence or toxicity after starting, stopping or interrupting CHC in a patient taking lamotrigine may reflect a clinically important bidirectional interaction requiring specialist review.

Assessment before treatment

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Treatment approach

01First-line unaffected optionsChoose IUD or DMPAFirst stepFirst lineA patient will use an enzyme-inducing medicine for a prolonged or uncertain duration.+
  1. 1Confirm the exact inducer, teratogenic risk, expected duration, current contraception, recent intercourse and medical eligibility.
  2. 2Offer copper IUD, LNG-IUD or standard-schedule DMPA as reliable methods unaffected by induction, comparing bleeding, procedure, bone and fertility-return effects.
  3. 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.+
  1. 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.

Regimens, contraindications and interactions

Provides reliable oestrogen-free contraception unaffected by enzyme-inducing antiseizure medicines, rifamycins or St John’s wort.

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.

Provides an oral emergency option when the preferred unaffected copper IUD cannot be used.

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.

Complications, monitoring and follow-up

  • 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.

Special situations

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.

Common pitfalls

  1. 01

    Checking interactions only for oral pills and missing that patch, ring and implant effectiveness can also fall.

  2. 02

    Stopping additional precautions on the last inducer dose instead of continuing for 28 days.

  3. 03

    Recommending standard-dose oral emergency contraception or ulipristal during recent enzyme induction.

  4. 04

    Doubling a POP, implant, patch or ring without evidence rather than offering an unaffected method.

  5. 05

    Calling lamotrigine an inducer and overlooking CHC-driven fluctuation in lamotrigine levels.

  6. 06

    Stopping essential antiseizure or antituberculous treatment instead of coordinating a safer contraceptive method.

Practice

Two practice questions

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Question 1 of 2
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Obstetrics and gynaecologyOriginal SBA

Unaffected long-term method

A patient using carbamazepine for epilepsy wants highly effective reversible contraception that will not be reduced by hepatic enzyme induction. Which option is appropriate?

Choose one answer, then submit
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Sources and review status4 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • CoSRH drug interactions guidanceInducer list, affected and unaffected methods, EC and lamotrigine.
  • CoSRH combined hormonal contraception guidelineCHC interaction management and specialist regimens.
  • CoSRH emergency contraception guidelineEmergency method selection with enzyme induction.
  • NHS combined pill interactionsPatient-facing recognition of medicines and herbal products that alter contraception.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom

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.
Pain, bleeding, collapse or a positive pregnancy test after an interaction requires urgent ectopic assessment when symptomatic and medicine-specific pregnancy advice.
  • Stopping an essential antiseizure, antituberculous or antiretroviral medicine to preserve contraception is unsafe; change the contraceptive plan with the relevant specialist.
  • 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.
  • 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.+
    1. 1Offer a copper IUD within the valid emergency window and assess pregnancy, infection, anatomy and consent promptly.
    2. 2If it is declined or unsuitable, avoid ulipristal and consider levonorgestrel 3 mg with transparent counselling that effectiveness evidence is limited.
    3. 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.+
    1. 1Clarify indication, seizure control, lamotrigine dose and hormone regimen and involve the specialist prescriber before any change.
    2. 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.
    3. 3Prefer a suitable non-CHC method when acceptable or coordinate dosing, regimen and monitoring explicitly across contraceptive and neurology or psychiatry care.