Those who had Alzheimer's at the start of the study were 69% less likely to be admitted to hospital with cancer than those free of the disease at the start.
And those with cancer at the study's start were 43% less likely to develop Alzheimer's than the cancer free.
There are biological reasons why this relationship could be causal:
"Alzheimer's disease and cancer are both characterised by abnormal, but opposing, cellular behaviour.
However, it is also possible that the symptoms of one disease may mask or prevent the diagnosis of the other disease, so more studies are necessary.