This invention relates to the use of CBD to treat mental disorders. For example it can be used as an adjunct therapy to treat positive symptoms in mental disorders such as schizophrenia and related disorders; or as an adjunct therapy in combination with olanzapine to treat both positive and negative symptoms in mental disorders such as schizophrenia and related disorders; or as an adjunct therapy in combination with olanzapine to treat negative symptoms in mental disorders such as schizophrenia and related disorders; or as an adjunct therapy to treat anhedonia / asociality in mental disorders such as schizophrenia and related disorders; or as an adjunct therapy to treat avolition / apathy in mental disorders such as schizophrenia and related disorders; or as an adjunct therapy to treat disturbance of attention in mental disorders such as schizophrenia and related disorders; or as a monotherapy or as an adjunct therapy to treat one or more distinct sub-domains of negative symptoms selected from : i) avolition/ apathy; ii) anhedonia / asociality and iii) disturbance of attention; or as an adjunct therapy to treat cognitive symptoms, particularly working memory, motor speed and executive function in mental disorders such as schizophrenia and related disorders. There may additionally be provided compositions comprising CBD in combination with one or more antipsychotics such as olanzapine and quetiapine. In particular the invention relates to the treatment of schizophrenia or related psychotic disorders, more particularly still those which are considered to be treatment resistant.