For a study, researchers sought to investigate the pharmacokinetic interactions of topiramate and etonogestrel in women who utilize contraceptive implants. They conducted a 12- to 36-month prospective, noninferiority research with healthy women utilizing etonogestrel implants. They evaluated baseline blood etonogestrel levels before starting a 6-week titrated topiramate regimen at conventional migraine (100 mg/day) and epilepsy (400 mg/day) doses. Serum etonogestrel concentrations were measured again after three weeks (100 mg/day), four weeks (200 mg/day), and six weeks (400 mg/day) of topiramate medication. They assessed etonogestrel in serum using a validated liquid chromatography-tandem mass-spectrometry assay and looked for noninferiority (a drop of less than 30%) from baseline.

They enrolled 48 individuals in all; 32 finished the three-week program, 31 completed the four-week program, and 27 completed all follow-up sessions. The median age of the participants was 25.3 (range 18.3–37.2), the median BMI was 25.5 kg/m^2 (range 18.7–42.2), and the median length of implant usage was 24 months (range 12–36). At baseline, median etonogestrel concentrations were 142 pg/mL (range 76.2–771), 126 pg/mL (range 72.4–585), 119 pg/mL (range 65.6–542), and 105 pg/mL (46.2–859). At 3 weeks, 4 weeks, and 6 weeks, the 95% CIs for mean percent change in serum etonogestrel concentrations from baseline were [-37.3% +16.9%], [-45.4% +5.2%], and [-66.8% +24.8%], respectively. With the exception of one person who had a serum etonogestrel concentration of less than 90 pg/mL at baseline, 30.8% of individuals (8/26, 95% CI 14.3–51.8%) had a serum etonogestrel concentration of less than 90 pg/mL at 6 weeks.

Despite being a mild enzyme-inducing antiepileptic, concomitant topiramate use resulted in lower serum etonogestrel concentrations among implant users, with a significant proportion reaching etonogestrel concentrations below the threshold for ovulatory suppression when taking antiepileptic topiramate dosages.