The present disclosure relates to a method for treating certain types of tumors or cancers, such as plexiform neurofibromas (PN), plexiform neurofibromas associated with neurofibromatosis type 1 (NF1-PN), by orally administering an effective amount of mirdametinib to the patient, where an amount of mirdametinib is administered on the first day of treatment to provide (i) an AUC0-tau less than 400 ng·h/mL, (ii) a Cmax no more than 40 ng/ml, or (iii) both.