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A single dose of a popular class of psychiatric drug used to treat depression can alter the brain’s architecture within hours, even though most patients usually don’t report improvement for weeks, a new study suggests.

More than 1 in 10 adults in the U.S. use these drugs, which adjust the availability of a chemical transmitter in the brain, serotonin, by blocking the way it is reabsorbed. The so-called Selective Serotonin Reuptake Inhibitors, or SSRIs, include Prozac, Lexapro, Celexa, Effexor, Paxil and Zoloft.

The findings could be a first step toward figuring out whether a relatively simple brain scan might one day help psychiatrists distinguish between those who respond to such drugs and those who don’t, an area of  mystery and controversy in depression treatment.

Researchers at the Max Planck Institute in Leipzig, Germany, used a magnetic resonance imaging machine to compare connections in the gray matter of those who took SSRIs and those who did not. They were particularly interested in what goes on when the brain is doing nothing in particular.

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