Donepezil (sold as "Aricept") has progression-slowing effects in established Alzheimer's disease and recently* in Mild Cognitive Impairment as well. However, even though the progression from MCI to AD was less for people treated with donepezil in the first 12 months, later, after 3 years, they had deteriorated as much as the placebo group.

* Petersen RC, Thomas RG, Grundman M, Bennett D, Doody R, Ferris S, et al Vitamin E and donepezil for the treatment of mild cognitive impairment. New England Journal of Medicine, 2005;352(23):2379-88.

Earlier Alzheimer's studies (Abstracts from PubMed):
The Gauthier S, Feldman H, Hecker J, Vellas B, Emir B, Subbiah P Functional, cognitive and behavioral effects of donepezil in patients with moderate Alzheimer's disease. Curr Medical Research Opinion, 2002;18(6):347-54

Objective: To investigate the efficacy and safety of donepezil in a subgroup of patients with Alzheimer's disease (AD) of moderate severity from a previous trial.
Methods:Two hundred and seven patients with moderate AD were randomized to treatment in this 24-week, double-blind, placebo-controlled trial. Patents received either donepezil, 5 mg/day for the first 28 days and 10 mg/day thereafter (n = 102), or placebo (n = 105). The primary outcome measure was the Clinician's Interview-Based Impression of Change with caregiver input at week 24.
Results: Least-squares (LS) mean sMMSE scores at baseline were 13.6 +/- 0.3 for the donepezil group and 13.9 +/- 0.3 for the placebo group. LS mean CIBIC-plus scores for donepezil-treated patients were improved from, or close to, baseline severity at all visits, and were significantly different from placebo at weeks 8, 12, 18, and 24 (week 24 LOCF mean difference = 0.53, p = 0.0003). LS mean change from baseline scores on the sMMSE and Severe Impairment Battery (SIB) for the donepezil group improved throughout the study, and were significantly different from placebo at each visit for the sMMSE (week 24 LOCF mean difference = 2.06, p = 0.0002) and from week 8 for the SIB (week 24 LOCF mean difference = -4.44, p = 0.0026). LS mean change scores on the Disability Assessment for Dementia remained at or above baseline levels throughout the study for the donepezil group, while the placebo group showed a steady decline; treatment differences were significant at each visit (week 24 LOCF mean difference = -9.25, p < 0.0001). LS mean change scores on the Neuropsychiatric Inventory 12-item total improved throughout the study for the donepezil group and were significantly different from placebo at weeks 4 and 24 (week 24 LOCF mean difference = 5.92, p = 0.0022). Eighty-one per cent of donepezil-treated and 89% of placebo-treated patients completed the trial, with 9% and 5%, respectively, discontinuing due to adverse events (AEs). Eighty-two per cent of donepezil-treated and 80% of placebo-treated patients experienced AEs, the majority of which were rated mild in severity and, in general, were similar between treatment groups.
CONCLUSION: The significant treatment responses observed with donepezil in these patients reinforce the findings from earlier studies that show donepezil to have important benefits, compared with placebo, across functional, cognitive, and behavioral symptoms, with good tolerability, in patients with AD of moderate severity.

Donepezil for mild and moderate Alzheimer's disease

Birks J S, Melzer D. Donepezil for mild and moderate Alzheimer's disease (Cochrane Review). In: The Cochrane Library, Issue 2, 1999. Oxford: Update Software.

ABSTRACT: The objective of this review is to assess whether or not donepezil improves the well-being of patients with mild or moderate Alzheimer's disease. Search strategy: The Cochrane Dementia and Cognitive Impairment Group Register of Clinical Trials, was searched using the terms 'donepezil', 'E2020' and 'ARICEPT'. Medline, PsychLIT and EMBASE electronic databases were searched with the above terms. Members of the Donepezil Study Group and Eisai Inc were contacted. Selection criteria: All unconfounded, double-blind, randomised controlled trials in which treatment with donepezil was administered for more than a day and compared with placebo in patients with Alzheimer's disease. Data collection and analysis: Data were extracted independently by the reviewers (JSB & DB), pooled where appropriate and possible, and the weighted or standardised mean differences or Peto odds ratios (95%CI) estimated. Where possible, intention-to-treat data were used.

Main results: There are 4 included trials, covering treatment of 12 or 24 weeks duration in highly selected patients. The only information available on one trial (Gauthier 1998) is a conference abstract which reports no usable results. Available outcome data cover domains including cognitive function and global clinical state, but data on several important dimensions of outcome are not available. The results of three trials suggest a small beneficial effect of donepezil in improving cognitive function: at a 5mg/day dose, improvements measured -2.6 points (95%CI -3.5 -- -1.8) on weighted mean difference, in the midrange of the 70 point ADAS-Cog scale. The results of two trials show some improvement in global clinical state (assessed by an independent clinician) in those treated with donepezil compared to placebo. The patients' own ratings of their Quality of Life showed no benefit of donepezil compared with placebo. There were significantly more withdrawals before the end of treatment from the 10mg/d (but not the 5mg/d) donepezil group compared with placebo, which may have resulted in some overestimation of beneficial changes at 10mg/d in progressively declining characteristics, as last available measures were used in analyses. A variety of adverse effects were recorded, but very few patients left a trial as a direct result of the intervention.

Reviewers' conclusions: In selected patients with mild or moderate Alzheimer's Disease treated for periods of 12 or 24 weeks, donepezil produced modest improvements in cognitive function and study clinicians rated global clinical state more positively in treated patients. No improvements were present on patient self-assessed quality of life, and data on many important outcomes are not available. The practical importance of these changes to patients and carertakers is unclear.