lamotrigine (LTG)in combination with valproic acid (VPA) may work synergistically to provide superior seizure control than each drug independently

VPA inhibits LTG metabolism by UGT1A4 glucuronidation, increasing plasma LTG concentrations^[1]

Valproic acid (VPA) decreases lamotrigine (LTG) clearance by 30% at 125 mg/day and 50% at 250 mg and higher^[2]

maximal inhibition of LTG clearance by VPA occurs at doses of 500 mg/day

when converting from VPA plus LTG therapy to LTG monotherapy: LTG may be kept steady until VPA is dropped to 125 mg/day, then raised by 30% and doubled on VPA discontinuation

when converting from LTG monotherapy to a polytherapy regimen with VPA, the dose of LTG can be cut by 50% (without affecting stable LTG concentrations) on the day that VPA is started at doses of 250mg/day or higher

when LTG is added to VPA, the risk of rash may increase unless LTG is introduced at a low dose and tapered up slowly but adding VPA per se to an LTG regimen does not increase this risk because the patient has already taken LTG long enough to become desensitised to this effect^[3]