Chances are someone you know is taking methylphenidate. The stimulant is prescribed for ADHD, and millions of people in the U.S. — an alarming number, according to some — are taking this medication or another stimulant to help them stay focused.

Teens who start using stimulant medications for ADHD for a short time in adolescence are at a higher risk of substance use.

Those who rely on these stimulants often talk quite openly about the difference the drugs have made in their daily lives.

The long-term impact, however, is a concern. That is why the results of a new University of Michigan study are intriguing. The news is both reassuring as well as a heads-up for parents: The research is believed to be the first national study to compare early use and longer-duration stimulant medication therapy with non-stimulant therapy for ADHD.

Among the more notable findings: Youth who take Ritalin, Adderall, or other stimulant medications for ADHD over an extended period of time early in life are no more at risk for substance abuse than teens without ADHD.

The researchers did find, however, that teens who start using stimulant medications for ADHD for a short time later in their adolescence — meaning during middle school or high school — are at a higher risk of substance abuse.

There was also no gender difference in the overall associations between stimulant medication therapy for ADHD and risk of substance abuse.

More than 40,000 people from 10 different sites took part in the nationwide study between 2005 and 2014, as part of the Monitoring the Future study.

Related: ADHD Drugs Could Affect Kids’ Bone Health

Given that higher substance-use behaviors are associated with later initiation of stimulant medications for ADHD during adolescence, the researchers recommend monitoring this later initiation subgroup carefully for pre-existing risk factors or the onset of substance use behaviors.

Teens had a 61 percent increased risk of heart arrhythmia during the first two months of use of the drug.

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And as a further heads-up for parents when it comes to Ritalin (methylphenidate): Another recent study has found that the medication may slightly increase the risk of an abnormal heart rhythm in children and teenagers who are prescribed the drug for treatment of ADHD.

A multinational team of researchers found that children and teens had a 61 percent increased risk of heart arrhythmia during the first two months of use of the drug. There was no significant increased risk for hypertension, ischemic stroke, or heart failure.

Patients with pre-existing congenital heart disease appear to be at greatest risk. Most children on the medication, however, should not experience heart problems.