One-fifth of patients with schizophrenia receive prescriptions for drugs that can cause dangerous interactions when taken in combination, a researcher said here.

Significant adverse effects were rare, but between 18% and 22% of Ohio Medicaid patients under treatment for schizophrenia were given prescriptions for an antipsychotic and one or more other drugs with well-known interaction potential by the same physician or pharmacy, reported Jeff Guo, Ph.D., of the University of Cincinnati.

What’s more, 11% to 12% of patients received prescriptions for such dangerous drug combinations from the same provider on the same day, Dr. Guo told attendees at the American Psychiatric Association meeting.

“That shouldn’t happen,” he said, because the interactions analyzed in the study have been described extensively in the medical literature.

Dr. Guo and colleagues analyzed Medicaid data on 27,909 patients in Ohio from 2000 to 2003, searching for simultaneous prescriptions of antipsychotic drugs with other medications that are known to have potentially dangerous interactions with the antipsychotic.

They found that more than 6,400 patients received potentially risky drug combinations.

The combinations included:

  * Aripiprazole (Abilify) with ketoconazole (Nizoral)
  * Clozapine (Clozaril) with ritonavir (Norvir)
  * Clozapine with fluvoxamine (Luvox)
  * Haloperidol (Haldol) with lithium
  * Risperidone (Risperdol) with fluoxetine (Prozac)

Comorbidities likely to require drug treatment were common, the data showed. Depression, anxiety, diabetes, and hypertension were each diagnosed in more than 10% of patients in the study.

Several comorbidities were significantly more common among patients receiving dangerous drug combinations according to multiple regression analysis. They included depression (P=0.001), substance abuse (P=0.0003), anxiety (P=0.0003), hyperlipidemia (P=0.008) and chronic obstructive pulmonary disease (P=0.028).

Dr. Guo said actual incidents of adverse events stemming from interactions were very uncommon. He and his colleagues identified 109 episodes among the 6,808 patients receiving risky drug combinations, or about 1.7%.

They defined adverse events as those known to have potentially severe or life-threatening consequences, such as extrapyramidal symptoms, increased seizure risk, QT interval prolongation, and arrhythmias.

Among the actual interaction-related adverse events identified in the study, the researchers found the average treatment cost was $10,511 over the 90 days following the episode’s onset.

The 90-day cost for other patients - including those who received risky combinations but did not have adverse effects - was less than $1,900.

Combinations with major interaction potential were prescribed simultaneously by the same clinician in 18.9% of patients. Pharmacies dispensed such combinations in 22.1% of patients.

In about half of these cases, the combinations were provided by the same clinician or pharmacy on the same day: 11.3% of patients received same-day prescriptions from clinicians and 12.2% had drugs dispensed on the same day from the same pharmacy.

“The take-home message to physicians is that we need to prevent these drug-interaction pairs, we need to educate physicians more,” Dr. Guo said.

David Baron, D.O., of Temple University in Philadelphia and program chair of the meeting, agreed that more should be done to prevent drug interactions.

“It’s not well taught in medical schools and is so important,” he said.

“The essence of this poster ... carries beyond schizophrenia,” he added. Dr. Baron argued that every physician who prescribes drugs needs to pay attention to the potential for drug-drug interactions.

He said technologies are now available that flag drug combinations with interaction potential, but they have only just begun to be incorporated into real-world practice.

“We’ve got a long way to go with that,” Dr. Baron said.

The study was funded by Ortho-McNeil Janssen Scientific Affairs. No potential conflicts of interest were reported.

Primary source: American Psychiatric Association, abstract # NR-04

By John Gever, Staff Writer,
Published: May 09, 2008
Reviewed by Dori F. Zaleznik, MD; Associate Clinical Professor of Medicine, Harvard Medical School, Boston.