The drug trial is part of the federal government’s first national plan to address Alzheimer’s, which was unveiled Tuesday by Kathleen Sebelius, the secretary for health and human services. The government took the unusual step of assigning $50 million from the current year’s N.I.H. budget to research considered too promising to wait, including the Colombia trial and a study on whether inhaled insulin can ease mild cognitive impairment, Dr. Collins said. Another $100 million is proposed for 2013, mostly for research, but also for education, caregiver support and data collection.

Success for the Colombia trial is, of course, no sure thing. Many trials fail, and Alzheimer’s research has so far found no treatment effective for more than several months. But experts say that trying drugs years before symptoms emerge could have greater potential because the brain would not yet be ravaged by the disease. The trial will be financed with $16 million from the National Institutes of Health, $15 million from private donors through the Banner Institute and about $65 million from Genentech, the drug’s American manufacturer.

The drug, Crenezumab, attacks amyloid plaques in the brain. If it can forestall memory or cognitive problems, scientists will know that prevention or delay is possible and appears to lie in targeting amyloid years before dementia develops. Many, but not all, Alzheimer’s researchers believe amyloid is an underlying cause of Alzheimer’s.

In 2010, The New York Times reported on the pervasiveness of dementia in this large Colombian family and scientists’ hopes of testing prevention drugs. But persuading pharmaceutical companies to invest took months. There are scientific and ethical issues involved with giving drugs to people who are healthy and people who live in a developing country, some of whom have little education, paltry incomes and longstanding superstitions about the disease they call La Bobera — the foolishness.

“The first thing I did was to ask myself the question, Are we taking advantage of these folks?” said Richard H. Scheller, Genentech’s executive vice president of research and early development. “The answer was clearly no.”

The risks, he said, are balanced by the fact that if nothing is done, “they’re going to get this terrible, terrible disease for sure.”

The few trials of prevention therapies — involving ginkgo biloba, women’s hormone replacement treatment and anti-inflammatory drugs — have involved people not guaranteed to get the disease. These therapies either failed or caused adverse side effects.