Inclusion Criteria:

1. Residential/Inpatients at recruitment to the study with a history of at least 2 weeks behavioural disturbance.
2. Alzheimer's Disease only as per McKhann Criteria + Hachinski Score<=4.
3. Moderately severe to severe Alzheimer's Disease (baseline MMSE </=19).
4. Clinically significant agitation that requires treatment.
5. Severity of agitation defined by Cohen Mansfield agitation inventory (CMAI) > /=45.
6. Age >/= 55.

Exclusion Criteria:

1. Memantine usage in the 4 weeks prior to the start of the study.
2. On Cholinesterase inhibitor for less than 3 months and not on a stable dose.
3. Anti-psychotic, anti-epileptic, antidepressant, benzodiazepine, lithium or hypnotic dosage alteration in the 2 weeks prior to the start of the study.
4. Antiparkinsonian medication.
5. Hypersensitivity to memantine or any of the excipients in the formulation.
6. Severe renal impairment.
7. Epilepsy, history of convulsions or seizure, or receiving any anti-epileptic treatment.
8. Concomitant usage of N-methyl-D-aspartate (NMDA) antagonists such as amantadine, ketamine or dextromethorphan.
9. Recent myocardial infarction, uncompensated congestive heart failure and uncontrolled hypertension.
10. Severe, unstable or poorly controlled medical illness.
11. Any disability that may interfere with the patient completing the study procedure.
12. Active malignancy.
13. Delirium, pain or any medical illness as a clear cause of agitation.
14. Any important drug interactions: Prohibited during study and in the 14 days preceding enrollment/inclusion are: Analgesic dextromethorphan, Dopaminergics- amantadine, Warfarin due to theoretical INR prolongation.