(CNN) Exposure to common medications used to treat attention deficit hyperactivity disorder has increased by more than 60% in US children and adolescents, according to a new study.

The study, published Monday in the journal Pediatrics, looked at all calls to US poison control centers for unintentional or intentional exposure to ADHD medications between 2000 and 2014 among children and adolescents. The researchers found that the number of calls increased from 7,018 in 2000 to 11,486 in 2014 -- a 64% increase.

According to the study's authors, "exposure" refers to the unnecessary ingestion, inhalation or absorption of these medications.

"What we found is that, overall, during that 15 years, there was about a 60% increase in the number of individuals exposed and calls reported to poison control centers regarding these medications," said Dr. Gary Smith , director of the Center for Injury Research and Policy of the Research Institute at Nationwide Children's Hospital and a leading author on the study.

The new research comes one day after Oliver North , the National Rifle Association incoming president, suggested on "Fox News Sunday" that the drug Ritalin -- which is commonly used to treat ADHD -- is partially responsible for the recent increase in gun violence in the US.

Of the approximately 156,000 calls received by poison control centers during the study period, approximately 82% were considered unintentional exposures, and 18% were considered intentional.

There were three exposure-related deaths.

"The finding that was most surprising was the proportion, and the severity, of the exposures among the adolescents that were due to intentional exposure. We had three deaths, and all three were in the teenage group," Smith said.

The researchers also found that the frequency of these exposures increased by 71% between 2000 and 2011 before dropping 6.2% between 2011 and 2014.

It is unclear why rates of exposure began to decline in 2011, according to Smith.

"During the early 2000s, there were a number of FDA warnings that came out" describing the potential adverse effects of these medications, Smith said. "But you would expect that to have affected numbers during the early 2000s. We saw it change later, and we don't know why. That would be a fascinating area for future research," Smith said.

Attention deficit hyperactivity disorder is a neurobehavioral condition characterized by an ongoing pattern of inattention, hyperactivity and/or impulsivity that interferes with functioning or development, according to the National Institute of Mental Health

ADHD is among the most common behavioral disorder among children or adolescents, according to Dr. Caleb Alexander , co-director of the Center for Drug Safety and Effectiveness at Johns Hopkins, who was not involved in the new study.

"It's one of the most common disorders diagnosed in children, and the rates of diagnosis have increased a lot over the past two decades," Alexander said.

"It's been diagnosed more frequently in recent years than historically, but there are unlikely to be large baseline changes in the prevalence of this disorder. The changes in diagnosis are probably more from evolving thresholds for diagnosis than true changes in the population," he added.

ADHD diagnoses among children in the United States more than doubled between 2005 and 2014, according to a 2017 study . Approximately 14% of all children in the US were diagnosed with the disorder in 2014, compared with 6.8% in 2005.

The researchers in the new study looked at exposure to four common medications used to treat ADHD: methylphenidate (e.g. Ritalin), amphetamine (e.g. Adderall), atomoxetine and modafinil. Approximately 46% of the exposures were due to methylphenidate and 45% to amphetamine.

The medications are considered stimulants with overexposure symptoms including agitation, tremor, increased heart rate, confusion and seizures, according to the Mayo Clinic

It is unclear why these medications are so effective in treating ADHD. However, models suggest that they function by increasing dopamine and norepinephrine in the prefrontal cortex of the brain, which is responsible for complex cognitive behaviors such as decision making and social control. Approximately 80% of those who use these medications see improvements in ADHD symptoms, according to a 2013 study

The increase in exposures "probably follows the increase in diagnoses," Smith said. "We know that these medications are highly effective in the treatment of ADHD. And as the number of diagnoses goes up, so follows the number of prescriptions."

The researchers in the new study also compared exposure to ADHD medications among three age groups: 0 to 5 years, 6 to 12 years and 13 to19 years. They found the reasons behind the exposures differed substantially among these groups.

In the youngest group, the majority of cases were due to unintentional exposure from exploratory behaviors; for children between 6 and 12, most exposures were due to therapeutic errors or accidentally taking multiple pills; and among children 13 to 19, more than 50% were due to intentional overexposure.

"These are stimulants, and they're used by teens for various reasons," Smith said. "Students, for example, might take it to get through a final exam. But like other stimulants, they might also take it because it gives them a high."

Of the nearly 150,000 calls to US poison control centers, approximately 10% resulted in serious medical outcomes, including prolonged and sustained symptoms of vomiting, agitation, increased heart rate, high blood pressure or even death.

The study looked only at calls to US poison control centers and did not include those individuals who went directly to the hospital or who contacted a health care provider directly, so the actual number of exposures is probably much higher, according to Alexander.

"It's important to note here that poison control data provide an important but very incomplete measure of nonmedical stimulant use," Alexander said. "The vast majority of nonmedical use is not ultimately reported to poison control centers. If you think of all the college kids out there misusing these medications, most are not calling 911 or the poison control centers."

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To prevent intentional or unintentional overexposure to these medications, Alexander also suggests that parents store them in a safe place and dispose of any unused medications.

"Unfortunately, just as with opioids, these medicines are far too accessible in bathrooms and bedrooms and kitchen cabinets all over the country," Alexander said. "There are increasing numbers of pharmacies and hospitals and health systems that are building take-back programs for individuals seeking to dispose of these medicines. So I think we'll see more of these in the coming years."