Medications that treat depression, known as SSRIs (selective serotonin reuptake inhibitors), don't always work immediately or at all for some patients. However, new research on how SSRIs affect the brain may make treatment for those suffering from depression more effective.

A single dose of an antidepressant can change the brain, according to a new study published in the journal Current Biology on Sept. 18. MRI scans of the study's subjects taken before and after an SSRI dose showed that changes in connectivity occurred within three hours of taking the drug.

These results were surprising to the authors of the study since researchers previously believed that antidepressants took weeks to kick in, not hours. "We were not expecting the SSRI to have such a prominent effect on such a short timescale or for the resulting signal to encompass the entire brain," said Julia Sacher, the lead author of the study, in an email to TIME.

In the study, researchers took MRI scans of 22 people who had not been diagnosed as depressed and had never taken antidepressants. The MRI scans measured the oxygenation of blood flow in the participants' brains. The researchers looked at how the number of connections between small blocks called voxels changed after a single dose of the SSRI. The connectivity in the majority of regions in the brain actually decreased, while the connectivity in the thalamus and cerebellum increased.

The researchers say that this discovery could lead to new ways of treating patients suffering from depression, such as being better able to determine if SSRIs will relieve patients from symptoms. It also opens the door for individualized therapy to be practiced more widely.

Still, researchers are far from understanding how different antidepressants affect people with and without depression after the first dose and in the long-run, Sacher told TIME. "The hope that we have for future studies is to uncover distinct differences in brain connectivity between depression patients who ultimately respond to an antidepressant and those who do not," she said.

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