There are an estimated 3.5 million children taking prescription medication to treat attention deficit disorder in the U.S.—but we know next to nothing about the long-term effects of taking these stimulants at a young age.

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However, last week, researchers at the University of Amsterdam released a study that uncovered at least one effect of childhood Ritalin use. Unlike people who started taking the drug after they turned 23, people who were prescribed Ritalin as children experienced long-lasting changes to the brain.

The study, published in the journal NeuroImage, showed "lasting changes" to the GABA neurotransmitter system among patients who started taking the drug while their brain was still developing, lead author Dr. Michelle Solleveld told PsyPost. "This was not the case when treatment was started after the age of 23, when the brain was matured."

Wait, what does the GABA neurotransmitter do?

When GABA neurotransmitters bind with neurons, they reduce the function of those cells. Research indicates that these neurotransmitters are responsible for mitigating fear or anxiety because they are "critical to keep the brain from being over-excited," Dr. Richard Olsen, a researcher at UCLA's Department of Molecular and Medical Pharmacology, told ATTN:.

The Ritalin study found that people who take the stimulant before their brains are fully developed have lower levels of GABA neurotransmitters, compared to people who take the drug after they turn 23. It also showed that Ritalin use causes a temporary spike in GABA levels for people who were prescribed the drug as children—but not for people who were prescribed Ritalin as adults.

This image shows the part of the brain associated with GABA neurotransmitters.

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"[R]educed GABA function will cause hyperexcitability—like anxiety, insomnia, even seizures," Olsen said. While he hasn't studied how Ritalin use affect GABA functioning, he said he'd "be surprised if long-term Ritalin did not cause brain changes, some of which would no doubt be bad for peoples' health."

The researchers behind the Ritalin study said that further research is needed to determine what kind of consequences this effect might have. They also called for more research to explore other long-term effects on the brain for people who are prescribed Ritalin as children, as the researchers focused exclusively on the GABA neurotransmitter system.

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"The prevalence of ADHD has increased rapidly over the last few years," Solleveld said. "With this, prescription rates of methylphenidate, also known as Ritalin, are increasing as well."

"The safety and efficacy of methylphenidate treatment in adults has been studied thoroughly," she added. "However, methylphenidate treatment for ADHD is actually more common in children, and no studies so far have investigated the possible long-term effects of methylphenidate on the developing brain."