Lamotrigine interactions with oral contraceptives

Ms. A, age 20, presents to the clinic after experiencing difficulty sleeping, depressed mood, fatigue, and difficulty concentrating. Her psychiatric history includes bipolar II disorder (BD II), predominantly with depressive episodes. Ms. A’s current medications include a combination of lamotrigine 200 mg/d and bupropion extended-release 450 mg/d, and her symptoms were well maintained until 2 weeks ago. When her psychiatrist performs a medication reconciliation at her medication management appointment, Ms. A indicates she started taking an oral contraceptive, ethinyl estradiol and norgestimate, approximately 1 month ago for management of endometriosis symptoms. She is not currently taking any other medications or supplements.

Lamotrigine is indicated for epilepsy and as maintenance treatment for BD I. It is also used off-label to treat other mood disorders. After oral administration, lamotrigine is rapidly and fully absorbed with a high bioavailability (98%). The principal metabolic pathway is via glucuronic acid conjugation, leading to the major inactive metabolite 2-N-glucuronide. Minor metabolites include 5-N-glucuronide and a 2-N-glucuronide metabolite.¹

Combined oral contraceptives contain an estrogen component, typically ethinyl estradiol, and a progestin component, which varies based on the specific formulation. The metabolism of ethinyl estradiol occurs through cytochrome P450 (CYP)3A4, CYP2C9, sulfation, and glucuronidation. For progestin—the second component of combined oral contraceptives and the lone com­ponent of progestin-only oral contraceptives—metabolism occurs via CYP3A4 and conjugation reactions.² This article focuses on lamotrigine interactions specifically with oral contraceptives, but it is important to note that other formulations of combined hormonal contraceptives, such as the combined contraceptive patch (Ortho Evra) and vaginal ring (NuvaRing), would be expected to interact in the same way as oral formulations.³

Bidirectional interaction

While many antiseizure medications are known to interact with and potentially decrease the efficacy of oral contraceptives (Table 1^3-6), the interactions between lamotrigine and oral contraceptives is uniquely bidirectional. Combined oral contraceptives are thought to interact with lamotrigine primarily via the estrogen component, which causes increased metabolism of lamotrigine through induction of glucuronidation. This drug interaction decreases the plasma concentrations of lamotrigine in the body by up to 2-fold, resulting in an increased risk of seizures or inadequate mood stabilization.¹ This effect on metabolism is very rapid, resulting in decreases in lamotrigine concentrations within 1 week.^4,7 A recent study suggested that certain progestins may also contribute to decreased plasma levels of lamotrigine, but the mechanism for this is unknown (Table 2^3-7).⁸

Clinicians should consider increasing the lamotrigine dose (potentially as much as 2-fold) in a patient who initiates treatment with a combined hormonal contraceptive. Dose increases should not be >50 to 100 mg/d every week.¹ Collect lamotrigine blood levels before starting a hormonal contraceptive and during dose titration. While there is not a well-established therapeutic range for lamotrigine in BD, expert consensus recommends a range of 1 to 6 mcg/mL.⁸

The lamotrigine dose should be decreased if combined hormonal contraceptives are discontinued. Dose decreases should not exceed 25% of the total daily dose per week.¹ Desogestrel, a progestin-only medication, may increase exposure to lamotrigine, but this has not been observed in research with other progestins.^5,9 When starting a progestin-only pill, monitor patients for signs of lamotrigine toxicity (ataxia, diplopia, dizziness) and consider monitoring their blood levels.

An important consideration to note with combined oral contraceptives is the hormone-free interval, also known as the pill-free week. Due to the rapid effect of estrogens, the lamotrigine concentrations have been shown to rise, even double, during this hormone-free interval, so patients should be closely monitored for adverse effects.³ Some recommend use of an extended cycle regimen (with a limited hormone-free interval), or continuous cycle regimen (with no hormone-free interval) to avoid fluctuations in lamotrigine levels.^3,5 Additionally, data suggest that in patients taking lamotrigine and valproate, which inhibits glucuronidation, oral contraceptives do not cause reductions in lamotrigine concentrations.^2,5 In these instances, dose increases of lamotrigine are not needed.

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