Inclusion Criteria:  Surgical intervention for chronic or subacute SDH Electrode strip placed at time of surgery (for clinical indication of enhanced seizure monitoring) Age 18-100 SD detected within 48h of surgery  Exclusion Criteria:  Acute SDH requiring large craniotomy (some acute subacute component within cSDH is acceptable) Strip not feasible or safe to place at conclusion of surgery Known or suspected infection Recurrent surgery Subjects who previously participated in the study (contralateral or recurrent hematoma) Severe renal impairment (CrCl 5-29) Use of other NMDA antagonist Severe hepatic impairment Child-Pugh Class C hepatic impairment Known allergy to memantine Memantine use at time of admission Inability to obtain enteral feeding (oral or via NGT) Patients on acetylcholinesterase inhibitors Women of child-bearing age who are unwilling to undergo dual contraception for 30 days.