STANFORD (CBS SF) — In the largest study of its kind, Stanford University researchers have found that the mental health impact of a school shooting will linger for years among students who were attending the facility where the gunshots rang out.

The study found that local exposure to fatal school shootings increased antidepressant use among youths.

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Specifically, the average rate of antidepressant use among youths under age 20 rose by 21 percent in the local communities where fatal school shootings occurred. And the rate increase — based on comparisons two years before the incident and two years after — persisted even in the third year out.

“There are articles that suggest school shootings are the new norm — they’re happening so frequently that we’re getting desensitized to them — and that maybe for the people who survive, they just go back to normal life because this is just life in America,” said said Maya Rossin-Slater, a faculty fellow at the Stanford Institute for Economic Policy Research. “But what our study shows is that does not appear to be the case. There are real consequences on an important marker of mental health.”

The study was published Monday by the National Bureau of Economic Research. It is the largest scale study to date on the effects of school shootings on youth mental health.

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“When we think about the cost of school shootings, they’re often quantified in terms of the cost to the individuals who die or are injured, and their families,” Rossin-Slater said. “Those costs are unfathomable and undeniable, but the reality is that there are many more students exposed to shootings and survive. And the broad implication is to think about the cost not just to the direct victims but to those who are indirectly affected.”

According to the Stanford study, more than 240,000 students have been exposed to school shootings in America since the mass slaying at Columbine High School in April 1999. And the number of school shootings per year has been trending up since 2015.

“We know that poor mental health in childhood can have negative consequences throughout life,” said study co-author Hannes Schwandt, an assistant professor at Northwestern. “At the same time, children are known to show significant levels of resilience, so it really wasn’t clear what we would find as we started this project.”

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The researchers examined 44 shootings at schools across the country between January 2008 and April 2013. Of those 44 school shootings, 15 of them involved at least one death. The 44 shootings occurred in 10 states: Alabama, California, Connecticut, Florida, Nebraska, North Carolina, Ohio, South Carolina, Tennessee, and Texas.