IMPORTANCE: Women with idiopathic generalized epilepsy (IGE) face challenges in treatment due to limited options that are both effective and safe. OBJECTIVE: To evaluate the effectiveness and safety of substitution monotherapy vs add-on therapy as second-line options for women who might become pregnant with IGE after failure of first-line antiseizure medications (ASMs) other than valproic acid. DESIGN, SETTING, AND PARTICIPANTS: Multicenter retrospective comparative effectiveness cohort study at 18 primary, secondary, and tertiary adult and children epilepsy centers across 4 countries, analyzing data from 1995 to 2023. Participants were women aged 10 to 50 years diagnosed with IGE who were prescribed a second line of ASM. MAIN OUTCOMES AND MEASURES: Treatment failure (TF), defined as the replacement or addition of a second ASM due to ineffectiveness, was compared between patients receiving ASM add-on or substitution monotherapy using inverse probability of treatment weighting (IPTW)-adjusted Cox proportional hazards regression. Exploratory analyses were also conducted to assess the effectiveness of individual ASMs and various ASM combinations. RESULTS: This study included 249 women with a median (IQR) age of 18.0 (15.5-22.0) years. Among them, 146 (58.6%) received an add-on regimen, and 103 (41.4%) received substitution monotherapy. During follow-up, TF occurred in 48 patients (32.9%) receiving add-on therapy and 36 (35.0%) using substitution monotherapy, with no significant differences between groups (IPTW-adjusted hazard ratio [HR], 0.89
95% CI, 0.53-1.51
P = .69). ASM discontinuation due to ineffectiveness or adverse effects occurred in 36 patients (24.7%) receiving add-on therapy and 29 (28.2%) receiving substitution monotherapy, showing no significant differences (IPTW-adjusted HR, 0.97
95% CI, 0.57-1.65
P = .92). Rates of ASM discontinuation due to adverse effects only were low in both groups, occurring in 13 patients (9.0%) receiving add-on therapy and 9 (8.7%) receiving a substitution monotherapy. Among add-on regimens other than valproic acid, the combination of levetiracetam and lamotrigine demonstrated a lower risk of TF compared with other combinations with levetiracetam plus other ASM (adjusted HR, 2.41
95% CI, 1.12-5.17
P = .02) and lamotrigine plus other ASM (adjusted HR, 4.03
95% CI, 1.73-9.39
P = .001). However, valproic acid remained the most effective second-line ASM when considering individual agents. CONCLUSIONS AND RELEVANCE: In this comparative effectiveness study of second-line treatment strategies for women with IGE, no significant differences were observed between substitution monotherapy and add-on therapy.