The purpose of double blind studies is to prevent bias due to demand characteristics or the placebo effect. In the case of investigating the effects of a drug that cures Alzheimer’s a double-blind study is beneficial compared to a single blind study. The participants are not aware if they received a placebo or a real drug, therefore their option on the treatment will less likely influence the outcome. In addition, the double-blind study eliminates the chance of the researcher affecting the outcome because they too are unaware of which participants received the placebo vs the real drug. In a single blind test, the researcher is aware of what is given to the participant therefore they can inadvertently reveal clues to the participant which might alter the outcome. The double-blind procedure is more of a precautionary stance to not affect the outcome of the research due to experimenter bias.

Reference:
https://www.verywell.com/what-is-a-double-blind-study-2795103