Disclosed are pharmaceutical compositions comprising mirtazapine and an anticonvulsant drug. Also disclosed are methods of preventing weight gain associated with the administration of mirtazapine comprising identifying a patient to whom mirtazapine is to be administered and administering to said patient a pharmaceutical composition comprising mirtazapine and an anticonvulsant drug. Further disclosed are methods of increasing the efficacy of mirtazapine comprising identifying a patient to whom mirtazapine is to be administered and administering to said patient a pharmaceutical composition comprising mirtazapine and an anticonvulsant drug.