Drug Interactions Between Antiseizure Medicines

Antiseizure medicines can change the concentrations and effects of other antiseizure medicines. The interaction may increase toxicity, reduce seizure control or require dose adjustment.

Pharmacokinetic versus pharmacodynamic interactions

Pharmacokinetic interactions change what the body does to a medicine—its absorption, metabolism or elimination.

Pharmacodynamic interactions change what medicines do together—for example, increasing sedation even when blood concentrations are unchanged.

Enzyme induction

Some older antiseizure medicines increase the activity of liver enzymes that metabolise other drugs.

Important enzyme inducers include carbamazepine, phenytoin, phenobarbital and primidone.

This can lower concentrations of other medicines and shorten their duration of action.

Induction is not identical between medicines

A 2024 systematic review and network meta-analysis confirmed that the degree of CYP3A induction differs between antiseizure medicines rather than forming a simple yes-or-no category.

Enzyme inhibition

Some antiseizure medicines inhibit metabolic pathways and can raise concentrations of other drugs.

Valproate is an important example because it can inhibit the metabolism of lamotrigine.

Valproate and lamotrigine

Valproate markedly slows lamotrigine clearance.

Lamotrigine must therefore generally be introduced more slowly and at lower doses when valproate is already being taken.

Rapid escalation increases the risk of serious skin reactions.

Carbamazepine autoinduction

Carbamazepine increases the activity of enzymes that metabolise carbamazepine itself.

Its concentration can therefore fall during the first weeks after treatment begins even if the dose remains unchanged.

Clobazam and cannabidiol

Cannabidiol can increase exposure to the active clobazam metabolite, N-desmethylclobazam.

This can increase sedation and may require dose adjustment.

Cenobamate

Cenobamate can inhibit CYP2C19 and induce CYP3A4.

It can therefore raise concentrations of some medicines such as phenytoin or phenobarbital while lowering concentrations of others.

Perampanel

Strong enzyme-inducing antiseizure medicines can lower perampanel concentrations.

This may affect effectiveness.

Oxcarbazepine and eslicarbazepine

These medicines generally have fewer enzyme interactions than carbamazepine but are not interaction-free.

Pharmacodynamic burden

Combining several sedating medicines can increase dizziness, falls, slowed thinking and fatigue even if no major blood-level interaction occurs.

Why polytherapy becomes complicated

When three or more antiseizure medicines are used together, adding or removing one drug can alter the effect of others.

This is one reason medication changes are often made gradually.

Stopping an inducer can raise other drug levels

If an enzyme-inducing medicine is withdrawn, metabolism of other medicines may slow.

Their concentrations can rise even though their own dose has not changed.

Therapeutic drug monitoring

Blood levels can help when an interaction is suspected, especially for medicines with complex pharmacokinetics or a narrow concentration–toxicity relationship.

The central point

Drug interactions are dynamic. Adding, removing or changing one antiseizure medicine can alter the exposure or side effects of another, so medication lists must be reviewed as a whole.

Sources

Evidence checked: September 2026.