Nexplanon is not 100% effective, but it comes extremely close. In the largest real-world study of the implant (the NORA study), the Pearl Index was 0.04, meaning roughly 4 pregnancies occurred per 10,000 women per year of use. That makes it the most effective reversible contraceptive available, but a small risk of pregnancy still exists.
How Effective Nexplanon Actually Is
The numbers behind Nexplanon are striking. In the NORA study, which tracked thousands of users, only 3 pregnancies occurred while the implant was in place. When researchers included pregnancies that happened within 7 days of removal, the total was 6. The resulting failure rate of 0.04 per 100 woman-years translates to better than 99.9% effectiveness.
For context, here’s how that compares to other methods. The birth control pill is about 99% effective with perfect use, but because people miss doses or take them late, real-world effectiveness drops to around 91%. IUDs are close to 100% effective but still have slightly higher failure rates than the implant. Nexplanon’s edge is that once it’s placed in your arm, there’s nothing to remember, take, or replace for years.
How the Implant Prevents Pregnancy
Nexplanon is a small rod inserted under the skin of your upper arm. It continuously releases a low level of a synthetic progesterone into your bloodstream, which does two things. First, it suppresses ovulation, so your ovaries don’t release an egg each month. This is the primary way it works. Second, it thickens cervical mucus, making it dense and sticky enough to block sperm from reaching the uterus. These two mechanisms working together are why the failure rate is so low.
The Implant Now Lasts Up to 5 Years
Nexplanon was originally approved for 3 years of use, but the FDA has since reviewed clinical trial data supporting extension to 5 years. In that trial, there were zero pregnancies during years 4 and 5, with a Pearl Index of 0 for both individual years and cumulatively. So the implant doesn’t appear to lose effectiveness as it ages, at least through the 5-year mark.
Does Body Weight Affect How Well It Works
This has been a reasonable concern. The original approval trial excluded women above 130% of their ideal body weight, so the label initially carried a warning about potentially reduced effectiveness in heavier women. A newer study enrolled a significant proportion of overweight and obese participants (38% of the study group) and found that effectiveness was consistent across BMI categories, age groups, and racial and ethnic backgrounds. However, because that study only covered 2 years, the FDA has been cautious about fully removing the weight-related warning from the label without longer-term data.
What Can Reduce Its Effectiveness
The rare pregnancies that do occur on Nexplanon are often linked to factors outside the implant itself. Certain medications speed up how your liver breaks down the hormone, lowering its levels in your blood. These include some seizure medications (like phenytoin, carbamazepine, and topiramate), the antibiotic rifampicin, the antifungal griseofulvin, and the herbal supplement St. John’s wort. If you take any of these, you’d need a backup non-hormonal method while on the medication and for 28 days after stopping it.
Timing of insertion also matters. If the implant is placed within the first 5 days of your period, it’s effective immediately. If it’s placed later in your cycle, you need to use condoms or avoid sex for the next 7 days while hormone levels build up.
Fertility Returns Quickly After Removal
One thing that surprises many people is how fast fertility bounces back. Over 90% of former implant users ovulate within 3 weeks of removal, and one study detected ovulation as early as 3 days after the rod came out. In terms of actual pregnancies, about 12% of women conceived within two weeks of removal, roughly 14% to 29% within 3 months, and nearly 96% within a year. The median time to pregnancy across studies ranged from about 3 to 9 months, which is comparable to stopping most other contraceptive methods.
So while Nexplanon is not technically 100% effective, the gap between its real-world performance and perfection is vanishingly small. The main scenarios where it falls short involve drug interactions or incorrect insertion timing, both of which are preventable.