Brief Answer:
Information

Detailed Answer:
Hello and welcome,

The lamotrigine and adderall will not change the effectiveness of the Nexplanon, but the Nexplanon hormone may alter the amount of lamotrigine that is active in your system. This is not a complete contraindication to combining these, but something to watch for.

The studies I saw that showed that contraceptive hormones can alter the level of lamotrigine were based on oral contraceptives ("the Pill") rather than on implantable hormonal contraceptives like Nexplanon, so it may be that the slow release of Nexplanon is ok for the lamotrigine.

You may want to double check with the neurologist or psychiatrist who prescribes your lamotrigine to make sure he/she is comfortable with the Nexplanon. I can't advise specifically as I don't know how severe your symptoms were or have been that require the lamotrigine.

I'm copying and pasting a report on the interaction of hormonal contraceptives (again, we are talking about oral rather than the slow release that occurs with implanted Nexplanon) for your reference if that helps:
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"Coadministration with estrogens or progestins may decrease the plasma concentrations and pharmacologic effects of lamotrigine due to induction of lamotrigine glucuronidation. One group of investigators cited seven suspected cases of this interaction in women treated with oral contraceptives that contained either ethinyl estradiol in combination with desogestrel or norethindrone or norethindrone alone. The contraceptives reduced plasma levels of lamotrigine by 41% to 64%, and a deterioration in seizure control was observed several days to two months after initiation of contraceptive use, necessitating an increase in lamotrigine dosage or discontinuation of the contraceptive. In some cases, contraceptive discontinuation led to lamotrigine toxicity that required dosage reduction. A pharmacokinetic study also reported similar reductions in lamotrigine plasma levels in patients on combination oral contraceptives, with lamotrigine clearance 2.5 times greater than in controls. The interaction is further supported by the fact that changes in hormone levels are known to influence the pharmacokinetics of glucuronidated drugs in humans, and elimination of lamotrigine is significantly increased during pregnancy. However, a population pharmacokinetics study in patients newly diagnosed with epilepsy and receiving oral lamotrigine monotherapy for up to 48 weeks found no significant effect of oral contraceptive use or dose on the oral clearance of lamotrigine. Lamotrigine also has been shown to have little or no effect on the pharmacokinetics of contraceptive hormones, although measurement of serum FSH, LH, and estradiol has indicated some loss of suppression of the hypothalamic-pituitary-ovarian axis. The clinical significance is unknown. Measurement of serum progesterone indicated no hormonal evidence of ovulation.

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