Repetitive transcranial magnetic stimulation (rTMS) has been broadly used as an antidepressant for unipolar depression, nevertheless it has not been fully studied for bipolar depression. Besides, few investigations compare the effect of stimulating over the right or left prefrontal cortex (PFC).

Objective
The aim was to compare the effect of rTMS applied over the right or left PFC of subjects with bipolar disorder (BD) using the Hamilton depression rating scale.

Methods
We recruited 12 patients with BD. They were randomized to receive rTMS over the right (1Hz) or left (5Hz) PFC. They all received one daily session, until complete 15. Changes in every item of Hamilton depression rating scale were compared between the two stimulation groups.

Results
There are no differences in the antidepressant effect of the stimulation over both, right or left PFC, when comparing final total scores with baseline. Comparison by items between groups showed that right stimulation induced a significant decrease respect to the baseline in the items: depressive mood, insomnia early, anxiety (psychological, and despersonalization and desrealization. Left stimulation induced a significant decrease respect to the baseline in the items: depressive mood, suicide, insomnia early, and work and activities.

Conclusions
Our results show that rTMS is an effective antidepressant for BD. This data suggest that the stimulating over the right or left PFC may have differential effects over certain clinical features. Further investigation with a bigger sample is needed to strength the relevance of selecting the laterality of stimulation for patients with different clinical features.