Just one dose of antidepressant medication can alter brain function, new research has found.

It was previously thought that the most popular class of antidepressant medication, selective serotonin re-uptake inhibitors (SSRIs), take up to a month to have an effect.

Now, research published in the journal Current Biology suggests that medication may alter brain connections much faster – in three hours.

In the UK, one in four people will experience a mental health problem in the course of a year, and mixed anxiety and depression is the most common disorder. Around 10% of adults in the US experience depression.

It is hoped the research will help doctors select the most appropriate treatment for patients.

For the study, researchers scanned the brains of 22 healthy people who were not depressed and had never taken antidepressants before. Scientists analysed the brain's "functional connectivity" to assess how synchronised brain activity is in different areas.

Three hours after participants took a single dose of SSRI, another brain scan revealed a dramatic change. The results showed widespread drop in connectivity throughout the brain, except where it was enhanced in two brain regions: the cerebellum and thalamus.

"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," study author Dr Julia Sacher, of the Max Planck Institute for Human Cognitive and Brain Science in Germany, told Live Science.

"The connectivity changes we report here are much more dramatic and acute than previous reports on SSRI-action in the human brain have indicated," she added.

SSRIs are widely prescribed for the treatment of depression, anxiety and some personality disorders. The medication blocks the reabsorption of the brain chemical serotonin, increasing the levels of free serotonin outside of cells.

It is believed that changes in serotonin levels may lead to reorganisation in the brain by affecting brain cell proliferation, the effectiveness of cell signal transmission, as well as other factors.

Previously, scientists thought it took several weeks for these changes to take place and the patient start to respond to the drugs.

"It is possible that these connectivity changes are the first step in remodeling the brain, as there is evidence from other experiments that such functional connectivity changes can reflect neuroplastic change," Sacher told Time.

Yet Sacher emphasised that further research is needed to find out how "context" affects medication on a patient's brain.

"Much work remains before we understand how different antidepressants affect the brains of people with and without depression, not only after the first dose, but also over the longer term," she said.

"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."

The researchers plan to conduct future research with people recovering from depression, as well as those who have taken SSRIs but have not benefited from the medication, to compare their changes in functional connectivity.