The mean steady-state plasma concentration of lamotrigine (LTG) was 13 micro mol/L in 22 women taking LTG in combination oral contraceptives (OC) compared with 28 micro mol/L among 30 women on LTG who did not take OC (p < 0.0001). The LTG dose/body weight/plasma concentration was 2.1 L/kg/day in women on OC compared with 0.8 L/kg/day in women without OC (p < 0.0001), indicating that LTG plasma levels are reduced by >50% during OC co-medication. It is advisable to monitor LTG plasma levels in conjunction with initiation or withdrawal of OC in women on LTG therapy.