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Deep brain stimulation can halve the symptoms of severe obsessive–compulsive disorder (OCD), according to a pooled data analysis of the available evidence, published in the Journal of Neurology, Neurosurgery and Psychiatry. The analysis found that two‐thirds of those affected experienced substantial improvement within two years. OCD is characterised by intrusive and persistent obsessive thoughts along with dysfunctional and ritualised behaviours. It is thought to affect up to 3% of people. Drugs and cognitive behaviour therapy can be very effective, but in approximately 1 in 10 cases, such approaches do not work. The researchers systematically reviewed and pooled the results of 34 clinical trials published between 2005 and 2021, to critically assess how well deep brain stimulation alleviates OCD and associated depressive symptoms in adults. The 34 studies included 352 adults with an average age of 40 years, and severe to extreme OCD, the symptoms of which had not improved despite treatment. In 23 of the studies, participants were required to have had persistent symptoms for five or more years before consideration for surgery. Of the remaining 11 studies, one had a requirement of more than a decade of symptoms and two or more years of failed treatment; another required at least one year of failed treatment; and five did not specify any requirements. On average, symptoms had persisted for 24 years. Coexisting mental health issues were reported in 23 studies and included major depression (over half of participants), anxiety disorder, and personality disorder. The average monitoring period after deep brain stimulation was two years. The final pooled data analysis, which included 31 studies involving 345 participants, showed that deep brain stimulation reduced symptoms by 47%, and two‐thirds of participants experienced substantial improvement within the monitoring period. Secondary analysis revealed a reduction in reported depressive symptoms, with complete resolution in nearly half of participants and partial response in a further 16%. Twenty‐four studies reported complete data on serious side effects, including hardware‐related complications, infections, seizures, suicide attempts, stroke, and the development of new obsessions associated with stimulation. Overall, 78 participants experienced at least one serious side effect.