Transcranial direct current stimulation (tDCS) is a promising neuromodulation method that has, for example, been used to treat depression. Nevertheless, the adverse effects of tDCS and the validity of the current standard tDCS sham protocols have received limited attention.

To evaluate the extent and types of tDCS adverse effects and to assess the reliability of sham stimulation as a control procedure for tDCS in a double-blind setting.

To compare adverse effects between tDCS and sham stimulation groups, and to determine how well the participants and the experimenter are able to distinguish tDCS from sham stimulation.

A sample of healthy volunteers received a 20-minute session of either tDCS (n=41; 2mA) or sham stimulation (n=41; ramp up 15s, ramp down 15s; no current in between). The anode was placed over F3 and cathode over F4. Both the participants and the experimenter reported immediate adverse effects and the perceived likelihood for the participant to receive tDCS. Analyses were conducted using the Mann–Whitney U-test.

The tDCS group reported more erythema compared with the sham group (P=0.016, Cohen's D=0.444). No other significant differences in adverse effects were observed. In the tDCS group, both the participants (P=0.034, Cohen's D=0.612) and the experimenter (P=0.006, Cohen's D=0.674) reported a higher perceived likelihood of the participant receiving tDCS than in the sham group.

tDCS has only modest adverse effects. Nevertheless, the current standard sham protocol appears insufficient.