Nexplanon releases a synthetic progestin called etonogestrel, a lab-made hormone similar to the progesterone your body naturally produces. The small, flexible rod contains 68 mg of etonogestrel total and slowly dispenses it into your bloodstream over the course of years, preventing pregnancy through three distinct mechanisms.
How Etonogestrel Leaves the Implant
The implant is a 4 cm rod made of a flexible plastic called ethylene vinyl acetate (EVA). The etonogestrel sits inside the core, surrounded by a thin EVA skin that controls how quickly the hormone seeps out into surrounding tissue and then into your blood. Think of it like a slow-release capsule, except it works for years instead of hours.
Hormone levels in your blood peak within the first two weeks after insertion, reaching roughly 1,200 picograms per milliliter. From there, the release rate gradually tapers. By 12 months, blood levels drop to about 202 pg/mL. By 24 months they’re around 164 pg/mL, and by 36 months they settle near 138 pg/mL. Even at that lowest point, the concentration remains high enough to prevent pregnancy effectively.
Three Ways It Prevents Pregnancy
Etonogestrel doesn’t rely on a single trick. It works through three overlapping mechanisms, which is part of why Nexplanon is one of the most effective contraceptives available.
- Suppressing ovulation. The primary effect. Etonogestrel signals your brain to stop triggering the hormonal surge that releases an egg each month. No egg, no pregnancy.
- Thickening cervical mucus. The hormone increases the viscosity of the mucus at the opening of your cervix, making it much harder for sperm to pass through and reach an egg.
- Altering the uterine lining. Etonogestrel changes the endometrium (the tissue lining your uterus) so that even if an egg were somehow fertilized, implantation would be far less likely.
Ovulation suppression does the heavy lifting. The other two mechanisms serve as backup layers, which is why the implant’s real-world failure rate is extremely low.
Why Hormone Levels Drop Over Time
The rod starts with a fixed supply of 68 mg and has no way to refill itself. As etonogestrel gradually diffuses out, less remains inside the core, so the release rate naturally slows. This is why blood levels in year three are roughly one-tenth of what they were in the first two weeks. The declining curve is expected by design and doesn’t mean the implant is failing. It simply releases more than it needs to early on, building in a margin that keeps it effective as levels taper.
FDA-Approved Duration: Now 5 Years
Nexplanon was originally approved for 3 years of use, but the FDA has since extended that to 5 years based on a clinical trial of 399 women who continued using their implants beyond the 3-year mark. During years 4 and 5, zero pregnancies were reported, giving a Pearl Index of 0.0 pregnancies per 100 women-years. The study included a wide range of body sizes, with 38% of participants having a BMI of 30 or higher and 10% having a BMI of 40 or higher. No new safety concerns appeared during the extended use period.
This means the hormone output at years 4 and 5, while lower than at insertion, still provides reliable contraception.
What Happens After Removal
Once the implant is taken out, etonogestrel clears your system quickly. The hormone has an elimination half-life of about 25 hours, meaning your body breaks down half of what’s circulating roughly every day. Within one week of removal, blood levels typically drop below detectable limits. Your body processes etonogestrel in the liver using the same enzyme pathway (part of the cytochrome P450 system) it uses to break down many other substances.
Because the hormone leaves so fast, fertility can return within days to weeks after removal. Most people ovulate again within a few weeks, though the exact timing varies from person to person.
How It Differs From Other Hormonal Methods
Nexplanon releases only a progestin. It contains no estrogen, which makes it distinct from combination birth control pills, patches, and rings. This progestin-only design is why it’s an option for people who can’t use estrogen due to migraine with aura, certain cardiovascular risk factors, or breastfeeding. The steady, low-dose delivery also avoids the daily hormone peaks and valleys that come with taking a pill, which some people find reduces side effects like nausea or mood swings.
The implant also contains barium sulfate, a substance that shows up on X-rays and CT scans. This isn’t a hormone, just a small amount of radiopaque material (15 mg) mixed into the rod so healthcare providers can verify its location in your arm at any time.