The purpose of this study is to determine if a medication called memantine is effective in
treating glioblastoma. Memantine targets a specific receptor, called a glutamate receptor,
which is thought to be involved in the growth of brain tumors. It has previously been studied
for other types of conditions, such as Alzheimer's disease, but it has not yet been evaluated
in the treatment of brain tumors. The investigators will also be determining how common it is
for patients with brain tumors to have side effects to memantine. Memantine will be taken by
mouth twice a day.