Eg. Combine the most promising 50 drugs, each at super low doses.

Low enough that side effects probably won't be bad, and if some prove entirely ineffective it doesn't matter.

But then, deliberately don't mix the ingredients precisely. Some people will get 10% more or less of each ingredient at random. Make precise records of this.

Now you have a natural experiment. Gradually increase dosage of elements that prove effective, and decrease ingredients which prove counterproductive - and you'll have a huge dataset with tens of millions of people wanting dementia treatment.