Background: An association between neuropsychiatric disorders and vitamin B12 deficiency has been recognized since 1849 when pernicious anaemia was first described. It has been suggested that deficiency of vitamin B12 might contribute to age-associated cognitive impairment. Low serum vitamin B12 concentrations are found in more than 10% of older people. A high prevalence of low serum vitamin B12 levels, and other indicators of vitamin B12 deficiency have been reported among people with Alzheimer's disease. A review is needed of trials assessing effects of vitamin B12 supplementation on cognitive function in later life.

Objectives: To examine the effect of B12 supplementation on cognitive function of demented and elderly healthy people in terms of preventing the onset or progression of cognitive impairment or dementia.

Search strategy: The trials were identified from a search of the Specialized Register of the Cochrane Dementia and Cognitive Improvement Group on 12 September 2002 using the terms listed in additional table 1. In addition MEDLINE 1966 to 2002/09 and EMBASE 1980-2002/08 were searched using the same terms and cognit* to pick up studies with healthy volunteers.

Selection criteria: All randomized double-blind trials in which vitamin B12 at any dose was compared with placebo.

Data collection and analysis: Both reviewers applied the selection criteria to assess the quality of the studies. One reviewer collated and analysed the data. For each outcome measure data were sought on every patient randomized.

Main results: From the two included studies (Seal 2002; Fourniere 1997) of people with dementia and low serum vitamin B12 levels, there was no statistically significant evidence of treatment effect, vitamin B12 supplementation compared with placebo, on cognitive function.

Reviewer's conclusions: Evidence of any efficacy of vitamin B12 in improving the cognitive function of people with dementia and low serum B12 levels is insufficient. The two trials of acceptable methodology (Fourniere 1997; Seal 2002) were restricted to a small number of patients with Alzheimer's disease and other types of cognitive impairment. No trials involving people without dementia or using other definitions of vitamin B12 deficiency were found.