Add-on stiripentol elevates serum valproate levels in patients with or without concomitant topiramate therapy.

•Add-on STP significantly increased serum VPA concentration.•Concomitant use of TPM augmented this effect.•Inhibition of the CYP2C19 pathway might be associated with these effects.•Marked increases in VPA concentrations may cause serious side effects.•Careful monitoring of VPA is necessary in patients concomitantly treated with TPM.