BACKGROUND: Folates are vitamins essential to the development of the central nervous system. Insufficient folate activity at the time of conception and early pregnancy can result in congenital neural tube defects. In adult life folate deficiency has been known for decades to produce a characteristic form of anaemia ("megaloblastic"). More recently degrees of folate inadequacy, not severe enough to produce anaemia, have been found to be associated with high blood levels of the amino acid homocysteine. Such degrees of folate inadequacy can arise because of insufficient folates in the diet or because of inefficient absorption or metabolic utilisation of folates due to genetic variations. Conventional criteria for diagnosing folate deficiency may be inadequate for identifying people capable of benefiting from dietary supplementation. High blood levels of homocysteine have been linked with the risk of arterial disease, dementia and Alzheimer's disease. There is therefore interest in whether dietary supplements of folic acid (an artificial chemical analogue of naturally occurring folates) can improve cognitive function of people at risk of cognitive decline associated with ageing or dementia, whether by affecting homocysteine metabolism or through other mechanisms. There is a risk that if folic acid is given to people who have undiagnosed deficiency of vitamin B12 it may lead to neurological damage. Vitamin B12 deficiency produces both an anaemia identical to that of folate deficiency but also causes irreversible damage to the central and peripheral nervous systems. Folic acid will correct the anaemia of vitamin B12 deficiency and so delay diagnosis but will not prevent progression to neurological damage. For this reason trials of folic acid supplements may involve simultaneous administration of vitamin B12. Apparent benefit from folic acid given in the combination would therefore need to be "corrected" for any effect of vitamin B12 alone. A separate Cochrane review of vitamin B12 and cognitive function is being prepared. OBJECTIVES: To examine the effects of folic acid supplementation, with or without vitamin B12, on elderly healthy and demented people, in preventing cognitive impairment or retarding its progress. SEARCH STRATEGY: Trials were identified from a search of the Cochrane Dementia and Cognitive Improvement Specialized Register Group on 9 April 2003 using the terms: folic acid, folate, vitamin B9, leucovorin, methyltetrahydrofolate, vitamin B12, cobalamin, cyanocobalamin, dementia, cognitive function, cognitive impairment, Alzheimer's disease, vascular dementia, mixed dementia and controlled trials. MEDLINE and EMBASE (both all years) were searched for additional trials on healthy people. SELECTION CRITERIA: All double-blind placebo-controlled randomized trials, in which supplements of folic acid with or without vitamin B12 were compared with placebo for elderly healthy people or people with any type of dementia or cognitive impairment. DATA COLLECTION AND ANALYSIS: The reviewers independently applied the selection criteria and assessed study quality. One reviewer extracted and analysed the data. In comparing intervention with placebo, weighted mean differences, and standardized mean difference or odds ratios were estimated. MAIN RESULTS: Four randomized controlled trials fulfilled the inclusion criteria for this review. One trial (Bryan 2002) enrolled healthy women, and three (Fioravanti 1997; Sommer 1998; VITAL 2003) recruited people with mild to moderate cognitive impairment or dementia with or without diagnosed folate deficiency. Fioravanti 1997 enrolled people with mild to moderate cognitive impairment or dementia as judged by scores on the Mini-Mental State Examination (MMSE) and Global Deterioration Scale and with serum folate level<3ng/l. One trial (VITAL 2003) studied the effects of a combination of vitamin B12 and folic acid on patients with mild to moderate cognitive impairment due to Alzheimer's disease or mixed dementia. The analysis from the included trials found no benefit from folic acid with or without vitamin B12 in comparison with placebo on any measures of cognition and mood for healthy or cognitively impaired or demented people: Folic acid effect and healthy participants: there was no benefit from of oral 750 mcg folic acid per day for five weeks compared with placebo on measures of cognition and mood of 19 healthy women aged 65 to 92. Folic acid effect and people with mild to moderate cognitive decline or dementia: there were no statistically significant results in favour of folic acid with or without vitamin B12 on any measures of cognitive function. Scores on the Mini-Mental State Examination (MMSE) revealed no statistically significant benefit from 2 mg per day folic acid plus 1mg vitamin B12 for 12 weeks when compared with placebo (WMD 0.39, 95% CI -0.43 to 1.21, P=0.35). Cognitive scores on the Alzheimer's Disease Scale (ADAS-Cog) showed no statistically significant benefit from 2 mg /day folic acid plus 1 mg /day vitamin B12 for 12 weeks compared with placebo (WMD 0.41, 95% -1.25 to 2.07, P=4.63). The Bristol Activities of Daily Living Scale (BADL) revealed no benefit from 2mg per day of folic acid plus 1 mg vitamin B12 for 12 weeks in comparison with placebo (WMD -0.57, 95%CI -1.95 to 0.81, P=0.42). None of the sub tests of the Randt Memory Test (RMT) showed statistically significant benefit from 15 mg of folic acid orally per day for 9 weeks when compared with placebo. One trial (Sommer 1998) reported a significant decline compared with placebo in two cognitive function tasks in demented patients who had received high doses of folic acid (10 mg /day) for unspecified periods. One trial (VITAL 2003) showed that 2 mg folic acid plus 1 mg vitamin B12 daily for 12 weeks significantly lowered serum homocysteine concentrations (P <0.0001). REVIEWER'S CONCLUSIONS: There was no beneficial effect of 750 mcg of folic acid per day on measures of cognition or mood in older healthy women. In patients with mild to moderate cognitive decline and different forms of dementia there was no benefit from folic acid on measures of cognition or mood. Folic acid plus vitamin B12 was effective in reducing the serum homocysteine concentrations. Folic acid was well tolerated and no adverse effects were reported. More studies are needed.