Antiseizure medications interactions

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Expected changes in plasma concentrations when an AED is added to a pre-existing regimen
AED added	PB	PHT	PRM	ETS	CBZ	VPA	OXC	LTG	GBP	TPM	TGB	LEV	ZNS	VGB	FBM
PB	-	PHT ↑↓	NCCP	ETS ↓↓	CBZ ↓↓	VPA ↓↓	H-OXC ↓	LTG ↓↓	↔	TPM ↓↓	TGB ↓↓	↔	ZNS ↓↓	↔	FBM ↓↓
PHT	PB ↑	-	PRM ↓ PB ↑	ETS ↓↓	CBZ ↓↓	VPA ↓↓	H-OXC ↓	LTG ↓↓	↔	TPM ↓↓	TGB ↓↓	↔	ZNS ↓↓	↔	FBM ↓↓
PRM	NCCP	PHT ↑↓	-	ETS ↓↓	CBZ ↓↓	VPA ↓↓	?	LTG ↓↓	↔	TPM ↓↓	TGB ↓↓	↔	ZNS ↓↓	↔	FBM ↓↓
ETS	↔	↔	NE	-	↔	VPA ↓	NE	NE	NE	NE	NE	NE	NE	NE	NE
CBZ	↔	PHT I↓	PRM ↓ PB ↑	ETS ↓↓	-	VPA ↓↓	H-OXC ↓	LTG ↓↓	↔	TPM ↓↓	TGB ↓↓	↔	ZNS ↓↓	NE	FBM ↓↓
VPA	PB ↑↑	PHT ↓*	PB ↑↑	ETS I↓	CBZ-E ↑	-	↔	LTG ↑↑	↔	TPM ↓	EE	↔	↔	NE	↔
OXC	PB ↑	PHT ↑	?	?	CBZ ↓	↔	-	LTG ↓	NE	?	?	NE	?	NE	?
LTG	↔	↔	NE	NE	↔	↔	NE	-	NE	NE	NE	↔	↔	NE	NE
GBP	↔	↔	NE	NE	↔	↔	NE	NE	-	NE	NE	↔	NE	NE	NE
TBP	↔	PHT ↑	↔	NE	↔	VPA ↓	?	?	NE	-	?	NE	?	NE	?
TGB	↔	↔	↔	NE	↔	↔	NE	NE	NE	NE	-	NE	NE	NE	NE
LEV	↔	↔	↔	NE	↔	↔	NE	↔	↔	NE	NE	-	NE	NE	NE
ZNS	↔	↔	NE	NE	CBA ↑↓	↔	?	↔	NE	NE	NE	NE	-	NE	?
VGB	PB ↓	PHT ↓	PRM ↓ PB ↓	NE	CBZ ↑	↔	NE	NE	NE	NE	NE	NE	NE	-	NE
FBM	PB ↑↑	PHT ↑↑	?	?	CBZ ↓ CBZ-E ↑	VPA ↑↑	↔	↔	NE	?	?	NE	?	↔	-

PB = Phenobarbital; PHT = phenytoin; PRM = Primidone; ETS = Ethosuximide; CBZ = Carbamazepine; VPA = Valproic acid; OXC = Oxcarbazepine; LTG = Lamotrigine

GBP = Gabapentin; TPM = Topiramate; TGB = Tiagabine; LEV = Levetiracetam; ZNS = Zonisamide; VGB = Vigabatrin; FBM = felbamate H-OXO = 10-hydroxy-oxcarbazepine(active metabolite of OXC); CBZ-E = Cabamazepine 10,11 epoxide NE = None Expected; * Free (pharmacologically active) concentration may increase; NCCP = not commonly coprescribed; ↔ = No change; ↓ = a minor (or inconsistent) decrease in plasma concentration; ↓↓ = a clinically significant decrease in plasma concentration; ↑ = a minor (or inconsistent) increase in plasma concentration; ↑↑ = a clinically significant increase in plasma concentration.

source:Patsalos PN, Perucca E (2003) Clinically important drug interactions in epilepsy: general features and interactions between antiepileptic drugs. Lancet Neurol 2 (6):347-56. DOI: 10.1016/s1474-4422(03)00409-5 PMID: 12849151