Plan B - Drug Monograph

Introduction

Plan B (levonorgestrel) is an emergency contraceptive medication used to prevent pregnancy after unprotected sexual intercourse or contraceptive failure. It contains a synthetic progestin hormone and is available over-the-counter without age restrictions in the United States.

Mechanism of Action

Plan B primarily works by:

• Inhibiting or delaying ovulation through suppression of the luteinizing hormone (LH) surge
• Altering the cervical mucus to create a barrier to sperm penetration
• Potentially affecting endometrial receptivity to implantation (though this mechanism remains controversial)

It is important to note that Plan B is not effective once implantation has occurred and does not terminate an established pregnancy.

Indications

• Prevention of pregnancy following unprotected intercourse or known/suspected contraceptive failure
• Recommended for use within 72 hours (3 days) after unprotected sex, though efficacy is highest when taken as soon as possible

Dosage and Administration

• Oral administration with or without food
• Should be taken as soon as possible within 72 hours after unprotected intercourse
• Can be taken at any time during the menstrual cycle

• No dosage adjustment required for hepatic impairment
• No specific recommendations for renal impairment
• Not recommended for repeated use within the same menstrual cycle

Pharmacokinetics

Contraindications

• Known hypersensitivity to levonorgestrel or any component of the formulation
• Confirmed pregnancy (as it is not effective and not indicated)

Warnings and Precautions

• Not intended for routine contraception
• Does not protect against HIV infection or other sexually transmitted diseases
• Ectopic pregnancy risk: Patients with severe abdominal pain should be evaluated for ectopic pregnancy
• Repeated use within the same cycle may disrupt menstrual patterns
• Decreased efficacy in women with BMI ≥ 25 kg/m² or weight > 165 lbs
• Not recommended for premenarchal girls

Drug Interactions

• Hepatic enzyme inducers (e.g., barbiturates, bosentan, carbamazepine, felbamate, griseofulvin, oxcarbazepine, phenytoin, rifampin, St. John's wort, topiramate) may decrease levonorgestrel concentrations
• Antiretroviral medications (certain protease inhibitors and non-nucleoside reverse transcriptase inhibitors) may affect levonorgestrel levels

Adverse Effects

• Nausea (23%)
• Abdominal pain (18%)
• Fatigue (17%)
• Headache (16%)

• Dizziness
• Breast tenderness
• Menstrual changes (earlier or later onset, heavier or lighter flow)
• Vomiting (requires repeat dosing if within 3 hours of administration)

• Allergic reactions
• Diarrhea

Monitoring Parameters

• Pregnancy test if expected menstrual period is delayed beyond 1 week
• Monitoring of menstrual patterns following administration
• Assessment for signs of ectopic pregnancy if symptoms occur

Patient Education

• Take as soon as possible after unprotected intercourse (within 72 hours)
• Does not provide ongoing contraception—use regular contraceptive methods after taking
• Does not protect against sexually transmitted infections
• May cause temporary menstrual irregularities
• Seek medical attention if vomiting occurs within 3 hours of taking the medication
• Contact healthcare provider if menstrual period is delayed beyond 1 week
• Not intended for routine contraceptive use
• Store at room temperature

References

1. FDA Prescribing Information for Plan B (levonorgestrel) tablet 2. American College of Obstetricians and Gynecologists. (2019). Emergency Contraception. Practice Bulletin No. 152. 3. Glasier AF, Cameron ST, Fine PM, et al. Ulipristal acetate versus levonorgestrel for emergency contraception: a randomised non-inferiority trial and meta-analysis. Lancet. 2010;375(9714):555-562. 4. Trussell J, Raymond EG, Cleland K. Emergency contraception: a last chance to prevent unintended pregnancy. Contemporary Issues in Contraception. 2016. 5. Curtis KM, Jatlaoui TC, Tepper NK, et al. U.S. Selected Practice Recommendations for Contraceptive Use, 2016. MMWR Recomm Rep. 2016;65(4):1-66.