The US Centres for Disease Control and Prevention estimates that 11% of American children between the ages of four and 17 – or 6.4 million – had been diagnosed with ADHD as of 2011. About half were taking drugs to treat the disorder.
"We have too few long-term studies on the effects of these medicines," Dr Sanford Newmark told Reuters Health. "There's a big, big gap in our understanding of what the effects of these medicines might be. It's worrisome."
A paediatrician from the Osher Centre for Integrative Medicine at the University of California, San Francisco, Newmark was not involved in the current study.
Researchers from the Mario Negri Institute for Pharmacological Research in Milan, Italy, found a dearth of long-term studies on side effects of stimulants such as Ritalin, which some kids take into adulthood.
Safe and well tolerated
An extensive search revealed just six studies, all funded by pharmaceutical companies, that followed a total of 3 000 children treated with ADHD medications in clinical trials and examined related side effects for at least 12 weeks.
"ADHD drugs seem to be safe and well tolerated according to the available data, but more studies are needed," lead author Dr Antonio Clavenna told Reuters Health in an email.
Decreased appetite, insomnia, headaches and stomach pain were the most common side effects reported in the studies, the authors write in the Archives of Disease in Childhood. Patients also experienced tics and mood swings.
But the studies reported only common side effects and may have missed unusual ones, like suicidal thinking and long-lasting erections, that have concerned US regulators, the authors write.
"In this regard, the safety profile of medications is not fully understood, and monitoring is needed," Clavenna said.
The Food and Drug Administration (FDA) in December issued a warning that ADHD stimulants such as Ritalin and Concerta may in rare cases cause prolonged and painful erections, some lasting so long they might require surgery. In 2005, after short-term studies showed an increased risk of suicidal thinking in children and adolescents, the FDA ordered a black-box warning on atomoxetine, known as Strattera.
Kids diagnosed with ADHD tend to have trouble paying attention and may have poor impulse control. Doctors frequently prescribe medications to increase focus and reduce impulsivity.
Some children improve without prescription drugs with sleep and dietary changes, behavioural therapy or extra help at school.
Newmark said he often sees children whose parents complain about ADHD medication side effects not reported in studies.
"One of the side effects they don't talk about that I see all the time is that these kids have personality changes," he said. "Parents say, 'The medicine is working, but he's just lost his spark,' or, 'She's just lost her joy.'"
Hundreds of clinical studies have reported that most of the side effects of drugs used to treat ADHD are mild and temporary, the authors of the current study write.
Evaluation of drug safety
In their review, the proportion of patients that stopped taking ADHD medication, which ranged from 8% to 25%, was the only adverse event measured in all six studies.
Most study participants who stopped taking the medication because of side effects stopped in the first year.
Because the studies investigated different side effects, the authors could not compare studies or treatment outcomes. They call the reporting of side effects "unsatisfactory" and write, "more should be done to improve the evaluation of drug safety".
In 2011, Danish researchers, after reviewing short-term studies examining adverse reactions to ADHD drugs, also called for long-term safety studies. They pointed out that drug manufacturers funded almost all of the studies, and a majority of the authors received contributions from the pharmaceutical companies producing the medications.
"The pharmaceutical company's job is to sell their product," Newmark said.
"It is very important for us as doctors to base our practice not on what the companies say but on objective evidence," he said. "It's easier and cheaper for the pharmaceutical companies to do short-term studies and say, 'Look, the medicine works.'"
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