Levonorgestrel
Levonorgestrel is a synthetic progestin used as emergency contraception (single 1.5mg dose), in combined oral contraceptives, hormonal IUDs and progestin-only pills. Most effective for emergency use within 72 hours of unprotected intercourse.
- Chemical formula
- C21H28O2
- CAS number
- 797-63-7
- ATC code
- G03AC03
- Molecular weight
- 312.5 g/mol
- Drug class
- Synthetic progestin (emergency contraception)
- Also known as
- d-Norgestrel, Plan B
What is it?
Levonorgestrel is a synthetic progestin in clinical use since the 1960s, present in oral contraceptives, hormonal intrauterine devices, progestin-only pills and emergency contraception. Its single most studied indication is emergency contraception after unprotected intercourse, where a single 1.5mg dose is given as soon as possible. Levonorgestrel is on the WHO Essential Medicines List and is widely available — over-the-counter for emergency contraception in many jurisdictions.
Mechanism of action
In emergency contraception, levonorgestrel works primarily by delaying or inhibiting ovulation when taken before the LH surge. It does not interrupt an established pregnancy and does not have abortifacient effects. In ongoing oral contraception, lower daily doses suppress ovulation, thicken cervical mucus and alter endometrial receptivity. In hormonal IUDs, the local effect on the endometrium is dominant, with minimal systemic levonorgestrel exposure.
Pharmacokinetics
After oral administration, levonorgestrel is rapidly absorbed with peak plasma concentrations within 1 to 2 hours. Bioavailability is essentially complete. The molecule is metabolised in the liver and excreted in urine and faeces, with a terminal half-life of approximately 24 to 32 hours. The single emergency contraceptive dose is rapidly cleared and has no long-term effect on fertility. Pharmacokinetics may be affected by enzyme-inducing medications and St John's wort, which reduce levonorgestrel exposure.
Indications
Levonorgestrel is approved for emergency contraception (single 1.5mg dose), regular oral contraception (combined with ethinylestradiol or as progestin-only), and as the active component of hormonal intrauterine devices for long-term contraception and treatment of heavy menstrual bleeding. The molecule is not approved for routine pregnancy interruption (it is not abortifacient) or for use as a regular contraceptive at the emergency dose.
Safety profile
Common adverse effects of emergency contraceptive use include nausea, headache, fatigue, dizziness, breast tenderness and menstrual cycle changes. These are typically transient. Repeated emergency contraceptive use is not harmful but is less effective than regular contraception. According to the prescribing information, levonorgestrel is generally well tolerated and is available over-the-counter for emergency contraception in many jurisdictions, reflecting its favourable safety profile.
Products containing this ingredient
Frequently asked questions
How effective is levonorgestrel emergency contraception? ▾
Effectiveness depends on time from intercourse to dose. Levonorgestrel reduces pregnancy risk by approximately 85% when taken within 72 hours after unprotected intercourse, with declining effectiveness over that window. According to clinical guidelines, the dose should be taken as soon as possible after the unprotected event. Some efficacy persists up to 120 hours but is lower. Levonorgestrel is less effective in women with higher BMI, where ulipristal acetate or copper IUD may be preferred.
Does levonorgestrel cause an abortion? ▾
No. Levonorgestrel acts primarily by delaying or preventing ovulation. It does not interrupt an established pregnancy and does not have abortifacient effects. According to the prescribing information and current clinical guidelines, if the dose is taken when implantation has already occurred, it has no effect on the developing pregnancy. This distinguishes emergency contraceptive levonorgestrel from medications used for medical termination of pregnancy.
Can I use levonorgestrel as regular contraception? ▾
Levonorgestrel is the active ingredient in many regular contraceptive products at much lower daily doses than the single emergency dose. Repeatedly using emergency contraception as a primary method is less effective than regular contraception, with higher pregnancy risk per cycle. According to clinical guidelines, women requiring ongoing contraception should be offered a regular method.
What if I vomit after taking levonorgestrel? ▾
If vomiting occurs within 2 hours of taking the dose, levonorgestrel may not have been absorbed adequately and another dose should be considered. According to the prescribing information, women who vomit within this window should consult a clinician or pharmacist about taking a repeat dose. Vomiting after 2 hours generally does not reduce efficacy because absorption is largely complete.
Are there contraindications for levonorgestrel emergency contraception? ▾
Levonorgestrel emergency contraception has very few absolute contraindications, reflecting its favourable safety profile. The main contraindication is known hypersensitivity. Existing pregnancy is not a contraindication for safety reasons (no abortifacient effect) but means the medication will not be effective. According to the prescribing information, ulipristal acetate or copper IUD may be more effective alternatives in higher-BMI women.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.