What is Levonorgestrel?
Levonorgestrel is a synthetic hormone that mimics the effects of the natural hormone progesterone [1.3.5]. It belongs to a class of medications known as progestins [1.2.1]. First patented in 1960, it has become one of the most widely used medications for contraception globally [1.4.7]. It is available in multiple forms, including as a standalone emergency contraceptive pill, in combination with estrogen for regular birth control pills, and as the active hormone in certain intrauterine devices (IUDs) and implants [1.3.5, 1.6.2]. As of 2020, it was the 323rd most prescribed medication in the United States [1.4.7]. Levonorgestrel works primarily by preventing or delaying ovulation (the release of an egg from the ovary) and by thickening cervical mucus to impede sperm movement [1.3.6, 1.4.7]. It is important to note that it does not terminate an existing pregnancy [1.2.2, 1.2.7].
How Levonorgestrel Works: The Mechanism of Action
The primary mechanism of action for levonorgestrel depends on when it is taken during the menstrual cycle. Its main function is to prevent pregnancy by interfering with ovulation [1.3.6]. By binding to progesterone and androgen receptors, it can slow the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. This, in turn, suppresses the luteinizing hormone (LH) surge that is necessary for an egg to be released from the ovary [1.3.6].
Other potential mechanisms include:
- Thickening Cervical Mucus: Levonorgestrel can make the mucus in the cervix more viscous, creating a barrier that makes it difficult for sperm to travel into the uterus and reach an egg [1.3.6].
- Altering the Endometrium: Some research suggests that it may alter the lining of the uterus (endometrium), making it less receptive to the implantation of a fertilized egg, although the relevance of this effect to its efficacy is a topic of ongoing discussion [1.2.3, 1.3.6].
The effectiveness of oral levonorgestrel as an emergency contraceptive is highest when taken as soon as possible after unprotected intercourse, ideally within 72 hours [1.2.1]. It is not intended for use as a regular form of birth control [1.2.1]. For long-term contraception, the slow, continuous release of levonorgestrel from an IUD provides highly effective pregnancy prevention for several years [1.2.6].
Forms of Levonorgestrel: Emergency vs. Long-Term Use
Levonorgestrel is formulated in different dosages and delivery systems for distinct contraceptive needs. The two main categories are emergency contraception and long-term, regular contraception [1.5.1].
Emergency Contraception (EC)
Often called the "morning-after pill," this form contains a higher, single dose of levonorgestrel (typically 1.5 mg) [1.5.2]. It is intended for occasional or backup use after unprotected sex or contraceptive failure [1.2.1].
- Brand Names: Plan B One-Step, Take Action, My Way, Aftera, and others [1.6.1, 1.6.5].
- How to Use: A single tablet taken by mouth as soon as possible within 72 hours (3 days) after unprotected intercourse [1.2.1]. Some studies show effectiveness up to 120 hours, but efficacy decreases with time [1.7.5, 1.7.6].
- Availability: Widely available over-the-counter (OTC) without a prescription in the United States for people of all ages [1.4.7].
- Effectiveness: Can reduce the chance of pregnancy by 75% to 89% if taken within the 72-hour window [1.5.2].
Long-Term Contraception
In these forms, levonorgestrel is used in lower, steady doses for ongoing pregnancy prevention. It is considered more effective than using EC repeatedly [1.5.3, 1.5.5].
- Hormonal IUDs: These T-shaped devices are placed in the uterus by a healthcare provider and slowly release levonorgestrel over several years [1.2.6]. Brands like Mirena, Kyleena, Liletta, and Skyla offer protection for 3 to 8 years, depending on the device [1.2.6]. They are over 99% effective at preventing pregnancy [1.3.6].
- Combination Oral Contraceptives: These daily pills contain a low dose of levonorgestrel combined with an estrogen (like ethinyl estradiol) [1.3.7, 1.6.3]. They work by consistently preventing ovulation.
- Progestin-Only Pills (Mini-pills): While less common, some progestin-only pills also use levonorgestrel to provide a daily contraceptive option, particularly suitable for those who cannot take estrogen [1.3.4].
| Feature | Emergency Contraception (e.g., Plan B) | Hormonal IUD (e.g., Mirena) | Combination Pills |
|---|---|---|---|
| Primary Use | Backup after unprotected sex [1.2.1] | Long-term, continuous contraception [1.2.6] | Daily, continuous contraception [1.3.7] |
| Active Ingredient(s) | Levonorgestrel (1.5 mg) [1.5.2] | Levonorgestrel (slow release) [1.2.6] | Levonorgestrel + Ethinyl Estradiol [1.3.7] |
| How It's Used | Single oral dose [1.2.1] | Inserted into the uterus by a doctor [1.2.6] | One pill taken orally every day [1.5.5] |
| Duration of Effect | One-time use per incident [1.5.5] | 3 to 8 years [1.2.6] | 24 hours (requires daily dose) [1.5.5] |
| Typical Effectiveness | 75-89% (when used correctly) [1.5.2] | >99% [1.3.6] | >99% (with perfect use), ~93% (typical use) |
| Availability | Over-the-counter (OTC) [1.4.7] | Prescription; requires insertion procedure [1.2.6] | Prescription only [1.3.7] |
Potential Side Effects and Risks
Like all medications, levonorgestrel can cause side effects. The side effects can vary depending on the dosage and form.
Common Side Effects of Emergency Contraception:
- Changes in menstrual bleeding (heavier, lighter, or irregular spotting) [1.2.1]
- Nausea and vomiting [1.2.1]
- Headache and dizziness [1.4.2]
- Fatigue [1.4.2]
- Breast tenderness [1.4.2]
- Lower abdominal pain or cramping [1.4.5]
These side effects are usually temporary and resolve within a couple of days [1.4.7]. If you vomit within two hours of taking an EC pill, you should contact a doctor as you may need another dose [1.2.1].
Common Side Effects of Hormonal IUDs:
- Irregular bleeding or spotting, especially in the first 3-6 months [1.4.1]
- Amenorrhea (cessation of periods) over time [1.4.1]
- Ovarian cysts, which are usually harmless and resolve on their own [1.4.3]
- Abdominal or pelvic pain [1.4.1]
- Acne, headache, or breast tenderness [1.4.3]
Serious risks are rare but can include pelvic inflammatory disease (PID), uterine perforation during insertion, or ectopic pregnancy if the device fails [1.4.1, 1.4.3]. It is important to remember that levonorgestrel does not protect against sexually transmitted infections (STIs), including HIV [1.2.1, 1.2.2].
Conclusion
Levonorgestrel is a versatile and effective synthetic hormone central to modern contraception. Whether used as a high-dose emergency pill for backup protection or as a low, steady dose in long-term methods like IUDs and daily pills, its primary function is to prevent pregnancy by disrupting ovulation and impeding sperm. It is not an abortion pill and will not harm an existing pregnancy [1.2.7]. While the emergency pill is a vital tool for preventing unintended pregnancies, it is less effective and not recommended for regular use compared to more reliable, long-acting methods [1.5.3]. Understanding the different forms, mechanisms, and side effects of levonorgestrel allows individuals to make informed decisions about their reproductive health in consultation with a healthcare provider.
For more information, you can visit MedlinePlus. [1.2.1]
