Nexplanon is one of the most effective contraceptives available, with a failure rate of essentially zero in clinical trials. But certain medications, supplements, and rare circumstances can lower the hormone levels it releases, potentially putting you at risk for unintended pregnancy. Here’s what actually reduces its effectiveness and what doesn’t.
How Nexplanon Works (and What Can Go Wrong)
The implant releases a steady stream of a progestin hormone called etonogestrel into your bloodstream. This hormone prevents pregnancy primarily by stopping ovulation. Your body breaks down etonogestrel using a specific liver enzyme. Anything that speeds up that enzyme’s activity causes your body to clear the hormone faster than the implant can release it, dropping your blood levels below the threshold needed to suppress ovulation. That threshold sits around 90 picograms per milliliter.
Most of the factors that reduce Nexplanon’s effectiveness work through this single mechanism: revving up that liver enzyme so it chews through etonogestrel too quickly.
Medications That Lower Hormone Levels
The biggest and best-documented threat to Nexplanon’s effectiveness is a class of drugs known as enzyme-inducing medications. These are most commonly certain anti-seizure drugs, but the category also includes some HIV treatments and a widely used herbal supplement.
Anti-Seizure Medications
Several epilepsy drugs are potent enzyme inducers. In a study of women using the contraceptive implant, adding carbamazepine dropped median etonogestrel concentrations from 158 pg/mL to just 51 pg/mL. Eight out of ten participants fell below the 90 pg/mL ovulation-suppression threshold. That’s a dramatic reduction, and Nexplanon’s prescribing information explicitly states the implant is not recommended for women who need long-term use of these drugs.
The anti-seizure medications that pose this risk include carbamazepine, phenytoin, phenobarbital, oxcarbazepine, and topiramate. If you take any of these, the implant may not protect you reliably.
Several other anti-seizure medications do not speed up hormone metabolism and are considered safe to use alongside hormonal contraception. These include valproic acid, lamotrigine, gabapentin, levetiracetam, and zonisamide, among others.
Certain HIV Medications
The HIV drug efavirenz has a striking effect on implant hormone levels. In a pharmacokinetic study, women taking efavirenz-based antiretroviral therapy had etonogestrel levels 82% lower than women not on HIV treatment. By 24 weeks of combined use, 95% of participants on efavirenz had hormone levels below the ovulation-suppression threshold. Nevirapine, another HIV medication that works through a similar mechanism, did not significantly affect etonogestrel levels, so the risk is not uniform across all antiretrovirals.
St. John’s Wort
This is the one that catches people off guard. St. John’s Wort, a popular over-the-counter supplement used for mild depression, is classified by the FDA as a strong inducer of the same liver enzyme that breaks down etonogestrel. Taking it alongside any hormonal contraceptive, including the implant, can speed up hormone metabolism and raise the risk of unintended pregnancy. Because it’s sold as a supplement rather than a prescription drug, many people don’t think to mention it to their healthcare provider or consider its interactions.
Other Medications to Be Aware Of
Beyond anti-seizure drugs, HIV medications, and St. John’s Wort, other enzyme-inducing drugs can pose the same risk. These include rifampin (used for tuberculosis) and certain antifungal medications. The common thread is always the same: if a drug is known to induce the liver enzyme that metabolizes steroid hormones, it can potentially undermine the implant. If you’re prescribed a new medication while using Nexplanon, it’s worth asking whether it falls into this category.
Does Body Weight Affect Effectiveness?
This is a common concern, and the answer is more reassuring than many people expect. Women with higher body weight do have somewhat lower circulating etonogestrel levels, which makes pharmacological sense since the same dose of hormone is distributed across more body mass. However, a systematic review published in BMJ Sexual & Reproductive Health found that the implant’s real-world effectiveness in women with overweight and obesity was comparable to its effectiveness across all weight groups.
Among women with a BMI under 25, three pregnancies were reported across the reviewed studies. Among women with a BMI of 25 to 30, zero pregnancies were reported. Among women with obesity (BMI 30 and above), only two pregnancies were identified across nine publications, yielding failure rates between 0.02 and 0.23 per 100 woman-years. These numbers are still extremely low. The prescribing information does mention the possibility of earlier replacement for women with higher body weight, but the clinical evidence so far does not show a meaningful drop in effectiveness through the full three-year duration.
Genetic Variations in Hormone Metabolism
A small percentage of people carry a genetic variant that causes them to produce an enzyme normally active only before birth. Carriers of this variant break down steroid hormones, including etonogestrel, faster than the general population. One study found these carriers had roughly 45% lower levels of certain hormone metabolites due to this accelerated processing. This is not something most people would know about unless genetically tested, but it could explain rare cases where the implant seems less effective without an obvious cause.
Improper Insertion
This isn’t about the implant becoming less effective once it’s in place. It’s about the implant never being properly placed to begin with. If the device isn’t correctly inserted just beneath the skin, it may not release hormone into your bloodstream at all. The prescribing information warns that a failed insertion can go unnoticed unless the implant is physically felt (palpated) right after the procedure. You should be able to feel the implant as a small rod under the skin of your upper arm. If you can’t feel it at any point, that’s worth getting checked, because an implant that isn’t there can’t protect you.
An implant inserted too deeply can also migrate, making removal difficult later. This doesn’t typically reduce its contraceptive effect while it’s releasing hormone, but it complicates the process when it’s time for replacement.
Does It Lose Effectiveness Before Three Years?
For most users, no. In fact, research suggests the implant remains highly effective well beyond its approved duration. A study published in Human Reproduction followed over 200 women who kept their etonogestrel implant for five years instead of three. Zero pregnancies occurred during years four and five. The overall five-year pregnancy rate was 0.6 per 100 woman-years, which is still far lower than most other contraceptive methods.
This doesn’t mean you should skip your replacement appointment, since hormone levels do decline gradually over time, and individual variation matters. But it does mean that if you’re a few months past the three-year mark, the implant hasn’t suddenly stopped working. Its effectiveness tapers rather than dropping off a cliff.
What Actually Matters
For the vast majority of Nexplanon users, the implant works exactly as advertised with no action required on your part after insertion. The situations that genuinely compromise it are specific and identifiable: enzyme-inducing medications (particularly certain anti-seizure drugs and efavirenz), St. John’s Wort, and failed insertion. Body weight, based on current evidence, does not meaningfully reduce effectiveness. If you’re not taking any interacting medications and you can feel the rod in your arm, Nexplanon remains one of the most reliable contraceptives available.