Lamotrigine's risk of serious skin reactions, including Stevens-Johnson syndrome, is linked to a high starting dose and rapid escalation. The labelled titration takes several weeks to reach a maintenance dose.
Valproate inhibits lamotrigine metabolism and roughly doubles its levels, so lamotrigine is started at a lower dose and titrated more slowly when valproate is also prescribed. Enzyme inducers such as carbamazepine and phenytoin do the opposite and need higher doses. If lamotrigine has been stopped for more than about five half-lives, restart from the beginning of the titration.
- Follow the labelled titration schedule; do not shortcut it.
- Use the reduced titration schedule when valproate is co-prescribed.
- Tell patients to report any rash, fever or mouth ulcers in the first two months.
- Restart titration from the beginning after a gap in treatment of several days.
- Estrogen-containing contraceptives lower lamotrigine levels; review dosing when they start or stop.
Why it matters
Most serious lamotrigine rashes follow a titration that went too fast or ignored valproate.
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