Nexplanon almost never fails. In clinical trials, zero pregnancies occurred among women using the implant, giving it a failure rate of 0 per 100 women-years. That makes it one of the most effective contraceptives available, with fewer than 1 in 100 users becoming pregnant per year in real-world use. When pregnancies do happen with the implant in place, the cause is almost always traceable to a specific problem: the implant wasn’t placed correctly, a medication interfered with it, or the person was already pregnant at the time of insertion.
What the Clinical Data Shows
The FDA’s clinical review of Nexplanon found no on-treatment pregnancies among 394 women tracked over thousands of menstrual cycles. The Pearl Index, the standard measure of contraceptive failure, was 0.0 for both year four and year five of use. For context, the pill has a real-world failure rate of about 7%, and condoms fail roughly 13% of the time with typical use. Nexplanon sits alongside IUDs at the top of the effectiveness chart, with both methods preventing pregnancy more than 99% of the time.
The implant now provides protection for up to five years, extended from its original three-year approval after studies confirmed it maintained its effectiveness in years four and five.
Why It Sometimes Fails Anyway
The rare pregnancies reported in implant users typically trace back to one of a few identifiable causes rather than the device itself losing effectiveness.
Insertion Problems
A large observational study tracking over 7,300 insertion procedures found that incorrect insertion, including cases where the rod wasn’t fully placed, was partially inserted, or ended up too deep, occurred at a rate of about 12.6 per 1,000 insertions. That’s roughly 1 in 80 procedures. If the implant isn’t properly positioned under the skin, it may not release the hormone consistently, or in rare cases, it may not have been inserted at all. You should be able to feel the rod under the skin of your upper arm after placement. If you can’t, tell your provider before relying on it for contraception.
Drug Interactions
Certain medications speed up how quickly your liver breaks down the hormone in Nexplanon, potentially dropping its levels low enough to allow ovulation. The main culprits include some anti-seizure medications (phenytoin, carbamazepine, topiramate, oxcarbazepine), the antibiotic rifampicin, the antifungal griseofulvin, and several HIV medications. The herbal supplement St. John’s wort, commonly taken for mood, also reduces the implant’s effectiveness through the same mechanism. If you take any of these regularly, you may need backup contraception or a different primary method.
Some HIV protease inhibitors can either increase or decrease hormone levels depending on the specific drug. If you’re on antiretroviral therapy, your provider should review your full medication list before or after implant placement.
Timing of Insertion
Nexplanon works immediately if inserted during the first five days of your period. If placed at any other point in your cycle, it takes about seven days to become fully effective. Pregnancies can occur during that gap if no backup method is used.
Does Body Weight Affect How Well It Works
This has been a long-standing question. The original Nexplanon approval trial excluded women over 130% of their ideal body weight, which led to a label warning about possible decreased effectiveness in heavier women. A newer two-year study enrolled a more representative group: 38% of participants were overweight or obese, contributing over 2,500 evaluable cycles. No pregnancies occurred in any weight group, and the FDA’s statistical review noted that efficacy results were consistent across BMI categories.
That said, the FDA has been cautious about fully removing the weight-related warning, noting that two years of data may not be enough to draw definitive long-term conclusions. For now, the implant appears to work well regardless of weight, but this is still an area where the evidence is building.
If Pregnancy Does Occur
On the very rare occasion someone conceives with Nexplanon in place, there is a higher chance that the pregnancy will be ectopic, meaning the fertilized egg implants outside the uterus, typically in a fallopian tube. This is a pattern seen with progestin-only contraceptives generally. However, a systematic review in the journal Contraception confirmed that the absolute risk of ectopic pregnancy is actually lower for implant users than for women using no contraception at all, simply because pregnancy itself is so unlikely.
The etonogestrel implant (Nexplanon’s active ingredient) had fewer ectopic pregnancies reported than older levonorgestrel-based implants.
Telling Pregnancy Apart From Side Effects
One challenge with Nexplanon is that several of its common side effects overlap with early pregnancy symptoms. The implant frequently causes missed periods, nausea, breast tenderness, back pain, mood changes, and weight gain. All of these can also signal pregnancy.
The most reliable way to tell the difference is a pregnancy test. If your bleeding pattern changes significantly from what’s been normal for you on the implant, or you develop new symptoms like persistent nausea combined with breast changes, a home test can give you a quick answer. Because ectopic pregnancy is a concern if conception does occur, sudden sharp pain on one side of your lower abdomen warrants prompt medical attention.
As a practical habit, some providers suggest keeping a few inexpensive pregnancy tests on hand. Given how effectively Nexplanon suppresses periods, an occasional test can provide peace of mind when the usual monthly signal isn’t there to reassure you.