Pharmaceutical compounds

1. A method of treating dementia or a cognitive deficit associated with dementia in a patient suffering from Alzheimer's disease, which method comprises administering to the patient a therapeutically effective amount of a 3,11b-cis-dihydrotetrabenazine or a pharmaceutically acceptable salt thereof, to thereby treat dementia or a cognitive deficit associated with dementia in the patient.

2. The method of claim 1 , wherein the 3,11b-cis-dihydrotetrabenazine, or a pharmaceutically acceptable salt thereof, is a 2S,3S,11bR isomer of 3,11b-cis-dihydrotetrabenazine having the formula (Ia):

or a pharmaceutically acceptable salt thereof.

3. The method of claim 1 , wherein the 3,11b-cis-dihydrotetrabenazine, or a pharmaceutically acceptable salt thereof, is a 2S,3R,11bS isomer of 3,11b-cis-dihydrotetrabenazine having the formula (Ib):

or a pharmaceutically acceptable salt thereof.

4. The method of claim 1 , wherein the 3,11b-cis-dihydrotetrabenazine, or a pharmaceutically acceptable salt thereof, is a 2S,3S,11bR isomer of 3,11b-cis-dihydrotetrabenazine having the formula (Ic):

or a pharmaceutically acceptable salt thereof.

5. The method of claim 1 , wherein the 3,11b-cis-dihydrotetrabenazine, or a pharmaceutically acceptable salt thereof, is a 2S,3S,11bS isomer of 3,11b-cis-dihydrotetrabenazine having the formula (Id):

or a pharmaceutically acceptable salt thereof.

6. The method of claim 1 , wherein the 3,11b-cis-dihydrotetrabenazine is a free base.

7. The method of claim 1 , wherein the pharmaceutically acceptable salt of 3,11b-cis-dihydrotetrabenazine is an acid addition salt.

8. The method of claim 1 , wherein the salt is a methane sulphonate salt.

9. The method of claim 1 , wherein the dementia arises from or is associated with accumulation of amyloid plaques in the brain.

10. The method of claim 1 , wherein the dementia is a form of dementia arising from an amyloidopathy.