Background Evidence from recent clinical trials suggests that transcranial direct current stimulation (tDCS) may have potential in treating neuropsychiatric disorders. However, the optimal frequency at which tDCS sessions should be administered is unknown.

Objective/Hypothesis This study investigated the effects of daily or second daily tDCS sessions on motor cortical excitability, over a 5-day period.

Methods Twelve healthy volunteers received daily or second daily sessions of tDCS to the left primary motor cortex over the study period, in a randomized, intraindividual crossover design. Motor cortical excitability was assessed before and after tDCS at each session through responses to transcranial magnetic stimulation.

Results Over a fixed 5-day period, tDCS induced greater increases in MEP amplitude when given daily rather than second daily. Analyses showed that this difference reflected greater cumulative effects between sessions rather than a greater response to each individual tDCS session.