implanon and Carbatrol Interactions

Interactions between implanon(etonogestrel) and Carbatrol (carbamazepine)

carbamazepine and etonogestrel (Moderate Drug-Drug)

ADDITIONAL CONTRACEPTION RECOMMENDED: Coadministration with certain anticonvulsants such as carbamazepine, oxcarbazepine, phenobarbital, phenytoin, and primidone may reduce the efficacy of contraceptive hormones. There have been numerous case reports of menstrual abnormalities (e.g., breakthrough bleeding, amenorrhea, irregular menses) and unintended pregnancy occurring in women who received oral contraceptives with anticonvulsants. The incidence of menstrual irregularities associated with this combination has been reported to be as high as 65% in some studies. The interaction stems from accelerated clearance of contraceptive hormones as well as decreased plasma concentrations of unbound (active) hormones due to induction of hepatic CYP450 enzymatic activity and hormone-binding globulin capacity by some anticonvulsants. Pharmacokinetic studies have found that normally recommended dosages of carbamazepine, phenobarbital, oxcarbazepine, and phenytoin can individually reduce the systemic exposure (AUC) of ethinyl estradiol and levonorgestrel by a third or more.

MANAGEMENT: Women using hormonal contraceptives should be advised of the risk of breakthrough bleeding and unintended pregnancy during concomitant therapy with enzyme-inducing anticonvulsants. Alternative or additional methods of birth control should be used during and for at least two weeks after short-term and 4 weeks after long-term (greater than 4 weeks) anticonvulsant therapy. If a combination oral contraceptive pill is used, a regimen containing at least 50 mcg of ethinyl estradiol per day or equivalent should be selected. Although breakthrough bleeding is not necessarily indicative of low ethinyl estradiol serum levels or increased risk of ovulation, some clinicians suggest that women who experience breakthrough bleeding during enzyme-inducing therapy may be prescribed an increased dose of ethinyl estradiol above 50 mcg daily by combining more than one formulation of contraceptive pill if necessary. For levonorgestrel emergency contraception in patients receiving enzyme-inducing therapy, a regimen consisting of a 1.5 mg dose as soon as possible (within 72 hours of unprotected intercourse) followed by a 0.75 mg or 1.5 mg dose twelve hours later has been recommended by some clinicians. An alternative is a single 2.25 mg dose as soon as possible following unprotected intercourse. However, there are no data on efficacy, compliance, or side effects with any of these regimens. No precautions or recommendations are available for women using hormone-releasing intrauterine systems, but a significant interaction with these systems is thought to be unlikely due to their local action. Injectable progestin-only contraceptives are also thought to be unaffected by enzyme-inducing drugs.