The research, published inTORONTO (Reuters) - Drugs used to treat schizophrenia  and depression linger in the brain long after they have left the bloodstream, so brain scans might be a better way than blood analysis to judge the correct dose, leading Canadian researchers said on Tuesday.  the most recent issue of the British journal Molecular Psychiatry, could lead to smarter prescriptions for patients who are notoriously forgetful at taking their much-needed medications.

"You cannot rely on blood levels to give you a complete time course picture of what is happening in the brain," said Dr. Shitij Kapur, head of schizophrenia research at the Centre for Addiction and Mental Health in Toronto, and one of the lead researchers on the study.

"We are not suggesting any patients start doing trials themselves, but this could lead to alternate forms of dosing, like injectables or once-a-month dosing."

At least 60 percent of schizophrenics have problems keeping to their prescribed drug regimens, which are either too onerous or cause unwanted side effects.

"Schizophrenics have several challenges, including cognitive impairment, and negative attitudes to taking medications. So if it were simpler, and less intrusive, the more likely they will stay with it," Kapur said.

The study analyzed two types of drugs used to treat schizophrenia, olanzipine and risperidone, and similar results were also found with common antidepressants.

Positron emission tomography (PET) technology revealed that brain tissues absorb these psychoactive drugs more effectively than blood tissue, calling into question the common practice of prescribing drugs based solely on plasma levels: especially for drugs that act on the brain.

Kapur doubts that other drugs classes, including those used for hypertension  or diabetes, would show the same effects in the brain because their effects are immediate, compared to the delayed effects of psychiatric medicines. The effects of insulin for example, can be traced in the blood to a "perfect science," he said.

The latest version of the antidepressant Prozac made by Eli Lilly is designed to be taken once a week, rather than daily, showing that drug manufacturers have also noticed the lingering effects of these types of drugs, said Kapur.

Kapur said his team's research stemmed from prior animal studies and schizophrenics' reports that they felt just fine after taking their medications only once a day at night, rather than twice a day.

The patients changed their dosing regimen to avoid feeling tired from a morning dose and to help them sleep at night, which gave anecdotal evidence that these drugs had lingering properties.

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