Patients taking drugs to treat psychiatric disorders including schizophrenia, depression and dementia may be taking too much medication, a study suggests.

A report published yesterday in the journal Molecular Psychiatry found that some antipsychotic medications linger in the brain longer than is indicated by blood samples.

The findings by a team of researchers affiliated with the University of Toronto raise questions about traditional prescribing procedures that are based on medication levels found in the blood.

"The drugs that were sometimes being prescribed twice a day, even three times a day, may be able to be given less frequently, based on these findings," said study co-author Dr. Gary Remington, who is director of the schizophrenia and continuing-care medication-assessment program at the Centre for Addiction and Mental Health.

"It would mean that people could take their medication less frequently and still maintain the same response."

Using positron-emission tomography, or PET scans, the scientists tracked olanzapine and risperidone -- North America's most commonly used antipsychotic drugs -- in the brains of 10 healthy volunteers and five patients being treated for schizophrenia.

The researchers found that medication in a pill taken on a Monday could be detected in the brain on Friday, although there was no trace of the drug in the blood.

Beyond prescribing practices, the findings could have implications for drug development, with a focus on what's happening in the brain rather than in the blood, Dr. Remington said.

And the findings could lead to cost cuts for patients and health-care insurers if less medication is prescribed, he added.

The implications are far-reaching but are being viewed skeptically by some physicians, the researchers said.

"Historically we wag our fingers and tell people to take their medication every day and don't forget it," Dr. Remington said. "This is a kind of reverse of what we've been saying for many years."

Dr. Remington is among a group of researchers examining whether antipsychotic medications taken once every few days could be as effective as medications taken daily.

While preliminary results suggest it's possible to cut dosages, Dr. Remington said, it is too soon for patients to request that their medications be changed.

The research was financed in part by a grant from pharmaceutical company Eli Lilly Canada.