The purpose of this study is to determine whether tDCS is effective as an add-on treatment in SSRI-resistant patients with obsessive and compulsive disorder

Total number of scheduled patients to be recruited: 46 We used the effect size of the study of Ruffini and colleagues (2009) in which rTMS was used to target the left orbito frontal cortex to calculate the number of patients to be recruited. With a risk alpha of 5%, a power of 80%, the number of patients to be included was 46 with an anticipated rate of 10% of lost to follow-up patients. (calculation was made on the http://www.spc.univ-lyon1.fr/mfcalc/NSN/assistant.htm)

Statistical analysis to address the primary and secondary objectives:

Chi squared test will be applied for quantitative data.

Inclusion Criteria:

• patients diagnosed with OCD according to DSM V criteria
• YBOCS score of 16 and above, stable (<20% of variation) during at least one month.
• have received for at least 12 weeks of which 6 weeks was either at the maximum tolerated dose or alternatively the manufacturer's recommended maxi- mum daily dose.
• age between 18 and 70
• voluntary and competent to consent

Exclusion Criteria:

• Major depressive disorder diagnosed with the DSM V criteria with a MADRS score >30 (actual)
• schizophrenia and other psychotic disorder
• drugs or alcohol abuse
• acutely suicidal
• significant head injury, or of any neurosurgical procedure with implanted material
• pregnant women