NEW YORK (Reuters) -- Magnetic stimulation of the brain (transcranial magnetic stimulation, or TMS), delivered either at high frequency or at low frequency, appears to be effective for some patients with major depression who do not respond to drug therapy, results of a new study suggest.

However, four weeks of treatment may be required before clinically meaningful benefits are realized, Dr. Paul B. Fitzgerald, at Monash University in Melbourne, Australia, and colleagues point out in the Archives of General Psychiatry.

Because direct comparisons between high-frequency and low-frequency TMS have not been published, the investigators tested the two strategies in 60 outpatients who had failed at least two courses of antidepressant medications.

The subjects were randomly assigned to high- or low-frequency treatment, or to sham treatment for two weeks. After two weeks, about half of the subjects in the sham group initiated TMS treatment.

At the end of two weeks, depression scores had declined to similar extents in the two active treatment groups. After two more weeks, score improvements averaged 48 percent.

Twelve patients who had received at least 10 sessions of TMS achieved a greater than 50% reduction in the depression scores. Treatment with TMS invariably resulted in improved performance in several measures of cognitive function.

TMS achieves results similar to those with electroconvulsive therapy, except for psychotic depression, for which electroconvulsive therapy is more effective, Fitzgerald told Reuters Health.

Although longer treatment times are required with TMS, costs are lower, and it "seems better in terms of side effects as it has no memory impairment or anesthetic-associated risks," he said.