Patients will be asked to take treatment for 26 weeks in the Horizon Trial. Patients will take tablets 3 times daily. One-half of the patients will receive dimebon (latrepirdine), the drug being studied, and the other half will receive placebo (sugar pill), which does not contain active medicine.

Patients and their main caregiver will be seen at their trial clinician's office about 8 times. At each visit, the clinician will assess the treatment's safety and how well the treatment is working, as well as interview the caregiver. Patients will receive trial-related medical care throughout the duration of the trial.

Continuing treatment on dimebon (latrepirdine)

All patients who complete the trial (through week 26), including those who received placebo during the trial, will be eligible to receive dimebon (latrepirdine) at no financial cost until it is approved for sale.