Combined results from two trials showed the drug, called solanezumab, slows the speed of mental decline by a third in those with mild-to-moderate disease.
A pharmaceutical firm which developed the drug has claimed that it helps clear the protein 'plaques' thought to cause Alzheimer's, The Telegraph reported.
Researchers in two trials found that the drug slowed the pace of cognitive decline by 34 per cent, over an 18-month period, compared to those given a placebo.
Rachelle Doody, professor of neurology at Baylor College of Medicine, who was involved in the research, described the finding as "encouraging".
"These results represent an important step for the medical, academic, and scientific communities in understanding brain amyloid as a target of Alzheimer's disease therapies," she said.
The result was obtained by combining data from two individual trials, which each contained about 1,000 people.
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In August the firm and patients' groups were left bitterly disappointed after results from the two parallel trials were released.
Each trial indicated the drug did have an effect - but the results were not as strong as hoped. It suggested a dim future for the eagerly anticipated drug.
The new analysis is therefore a welcome boost for those with the disease.
However, it is still unclear whether Eli Lilly will apply to drugs regulators in the US and Europe to get solanezumab approved, the report said.
The results were presented at a meeting of the American Neurological Association in Boston, Massachusetts.