The antidepressant citalopram lowered by 37 percent the amount of amyloid beta, the main ingredient in brain plaques, in a selection of 23 healthy adults aged 18-50.
The findings were published in the journal Science Translational Medicine.
Citalopram, a common selective serotonin reuptake inhibitor (SSRI) that was approved for the US market in 1998, also stopped the growth of existing brain plaques in mice with a model of Alzheimer's disease and cut new plaque growth 78 percent.
Previous research has found some elderly people who have taken antidepressants also have less amyloid plaque than people who have not taken the mood-enhancing drugs.
Still researchers cautioned that they have not yet found evidence of a cause and effect.
"We are a long way from making a statement regarding the ability of SSRIs to prevent the cognitive decline associated with AD," said lead author Yvette Sheline, professor of psychiatry, radiology and neurology at the University of Pennsylvania.
The patients in this study were followed for a 24 hour period. Future research will focus again on elderly people who take antidepressants, and follow them for a two-week period.
"If we see a drop in levels of amyloid beta in their spinal fluid after two weeks, then we will know that this beneficial reduction in amyloid beta is sustainable," Sheline said.
Alzheimer's disease affects five million people in the United States, where it is the sixth leading cause of death.
The number of people with Alzheimer's is expected to grow in the coming decades as population expands and more people live to old age.