PLAN B
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PLAN B (PLAN B).
Levonorgestrel, a progestin, prevents pregnancy primarily by inhibiting ovulation and altering cervical mucus to impede sperm penetration. It may also inhibit implantation.
| Metabolism | Primarily hepatic via CYP3A4; also involves glucuronidation and sulfation. Metabolites include tetrahydro- and hexahydro-levonorgestrel. |
| Excretion | Renal (approximately 50% as unchanged drug and metabolites); fecal (approximately 40% as metabolites); less than 1% biliary. |
| Half-life | Terminal elimination half-life: 24-30 hours. Clinical context: The prolonged half-life supports single-dose regimen for emergency contraception; may be affected by obesity (shorter half-life in obese women). |
| Protein binding | Approximately 97-99% bound to albumin. |
| Volume of Distribution | Approximately 1.5-2.0 L/kg. Clinical meaning: Extensive distribution into tissues, including breast milk. |
| Bioavailability | Oral: Approximately 100% (levonorgestrel is a synthetic progestin with high oral bioavailability). |
| Onset of Action | Oral: Inhibition of ovulation occurs within 2 hours of administration if taken before the LH surge; contraceptive effect begins within 24 hours. |
| Duration of Action | Duration of contraceptive effect: Approximately 120 hours (5 days). Clinical note: Effective if taken within 72-120 hours of unprotected intercourse; efficacy declines with time. |
One 1.5 mg tablet (levonorgestrel) orally as a single dose, taken as soon as possible within 72 hours of unprotected intercourse.
| Dosage form | TABLET |
| Renal impairment | No dosage adjustment required for any degree of renal impairment, as levonorgestrel is primarily metabolized in the liver. |
| Liver impairment | Contraindicated in severe hepatic impairment (Child-Pugh class C). For mild to moderate impairment (Child-Pugh A or B), use with caution; no specific dose adjustment guidelines available. |
| Pediatric use | For females of childbearing potential (any age post-menarche), dosing is the same as adults: one 1.5 mg tablet orally as a single dose within 72 hours of unprotected intercourse. |
| Geriatric use | Not applicable for postmenopausal women; for women of reproductive age, dosing is the same as younger adults: one 1.5 mg tablet orally as a single dose within 72 hours of unprotected intercourse. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for PLAN B (PLAN B).
| Breastfeeding | Levonorgestrel is excreted into human breast milk in small amounts. The relative infant dose is estimated to be less than 5% of the maternal weight-adjusted dose, with an M/P ratio of approximately 0.1-0.2 based on limited pharmacokinetic data. Short-term use as an emergency contraceptive is considered compatible with breastfeeding. The American Academy of Pediatrics lists levonorgestrel as a compatible medication during breastfeeding. No adverse effects in nursing infants have been reported. |
| Teratogenic Risk | Levonorgestrel, the active ingredient in Plan B, is a progestin-only emergency contraceptive. Clinical data do not suggest a teratogenic effect when used inadvertently during early pregnancy. The drug acts primarily via inhibition or delay of ovulation and may alter tubal transport and endometrial receptivity. There is no evidence of increased risk of major malformations, spontaneous abortion, or fetal harm from exposure during the first trimester. Use is contraindicated in confirmed pregnancy due to lack of therapeutic benefit, but does not warrant termination based on exposure alone. No specific fetal risks have been identified for second or third trimester exposure as the drug is not indicated for use during established pregnancy. |
■ FDA Black Box Warning
None
| Serious Effects |
["Known pregnancy","Severe hepatic disease","Hypersensitivity to levonorgestrel"]
| Precautions | ["Not effective for established pregnancy","Reduced efficacy in women with BMI > 26 kg/m²","Menstrual irregularities","Use with caution in hepatic impairment","Contraindicated with severe hepatic disease","Potential interaction with CYP3A4 inducers"] |
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| Fetal Monitoring | Routine clinical monitoring is not specifically required for Plan B use. However, pregnancy should be ruled out if the patient presents with a missed menses or if the subsequent menstrual period is delayed by more than 7 days. In cases of suspected pregnancy, a urine or serum hCG test is indicated. For women with known cardiovascular risk factors, thrombophilia, or severe liver disease, no specific fetal monitoring is mandated, but clinical assessment for potential adverse effects (e.g., breakthrough bleeding, nausea) is standard. |
| Fertility Effects | Levonorgestrel emergency contraception does not impair long-term fertility. It prevents ovulation or fertilization in the current cycle and has no effect on subsequent cycles. After a single use, menstrual cycle may be disrupted, resulting in earlier or later menses. Return to normal fertility occurs immediately in the next cycle. There is no evidence of reduced fertility or delayed conception with repeated use, although such use is not recommended due to increased risk of menstrual irregularities. |