Atlanta—A surprising percentage of older Americans on antiepileptic drugs (AEDs) also use nonepilepsy drugs that could create significant pharmacokinetic interactions.

That’s according to a new study in the journal Epilepsia, which sought to determine the frequency with which older Americans with epilepsy who received questionable drug combinations, as well as assessing whether racial/ethnic, socioeconomic, and demographic factors played a role.

To do that, Emory University–led researchers conducted a retrospective analysis of 2008-2010 Medicare claims for a 5% random sample of beneficiaries aged 67 years and older in 2009; the data was augmented for minority representation.

The study found that interacting drug combinations affecting efficacy of the nonepilepsy drugs were found in 24.5% of incident and 39% of prevalent cases. At the same time, combinations affecting AED efficacy were determined in 20.4% of incident and 29.3% of prevalent cases.

Among factors predicting higher interaction risk were having one or more comorbidity, being eligible for a Medicare Part D low-income subsidy, and not living in the northeastern United States, according to study authors. Conversely, protective factors were Asian race/ethnicity, and treatment by a neurologist, they said.

“A substantial portion of older epilepsy patients received NED-AED combinations that could cause important [pharmacokinetic] interactions,” the researchers conclude. “The lower frequency among incident vs. prevalent cases may reflect changes in prescribing practices. Avoidance of interacting AEDs is feasible for most persons because of the availability of newer drugs.”

A 2010 study in Current Neuropharmacology reported that older AEDs “are susceptible to cause induction (carbamazepine, phenobarbital, phenytoin, primidone) or inhibition (valproic acid), resulting in a decrease or increase, respectively, in the serum concentration of other AEDs, as well as other drug classes (anticoagulants, oral contraceptives, antidepressants, antipsychotics, antimicrobial drugs, antineoplastic drugs, and immunosuppressants).”

That study also noted that “the serum concentrations of AEDs may be increased by enzyme inhibitors among antidepressants and antipsychotics, anti-microbial drugs (as macrolides or isoniazid) and decreased by other mechanisms as induction, reduced absorption or excretion (as oral contraceptives, cimetidine, probenecid and antacids),” but generally found fewer interactions with the newer drugs.