The Food and Drug Administration has proposed lowering the bar for approving drugs to treat people at the earliest stages of Alzheimer’s disease, before they have developed any serious impairment or overt dementia. The goal is commendable — to find ways to prevent or slow the progression of this terrible disease before it can rob people of their mental capacities. But the proposal raises troubling questions as to whether the agency would end up approving drugs that provide little or no clinical benefit yet cause harmful side effects in people who take the medications for extended periods.

Efforts to find effective treatments for more advanced stages of Alzheimer’s have been largely discouraging, so researchers and drug companies have increasingly focused on finding and treating patients before there is too much irreversible injury to the brain. Before approving drugs to treat patients who already have obvious dementia, the F.D.A. usually (but not always) requires manufacturers to demonstrate in clinical trials that the drugs will have a positive effect on preserving both mental capacities and the ability to function in daily life. No drug on the market has been able to slow a patient’s deterioration for more than a few months.

Now the agency is suggesting that there may be no practical way to demonstrate that a drug can preserve the ability to function in these very early-stage patients during the course of a clinical trial of “reasonable” duration. That proof would require a large, long-lasting and expensive clinical trial. Thus manufacturers with persuasive evidence may be allowed instead to rely on very subtle changes in a patient’s performance on cognitive tests and on biological markers (not yet identified) that measure the course of disease.

The proposal could stimulate researchers to come up with new ways to identify and test treatments in patients with early Alzheimer’s. But there are also risks to this approach. A small decline in scores on cognitive tests may have no bearing on whether patients will progress to serious disease. The test scores might also mistakenly identify people who are not in the early stages of Alzheimer’s and who, if treated, would suffer adverse side effects without receiving any clinical benefit.