Patients with schizophrenia have an increased risk of venous thromboembolism (VTE), and this might be associated with the use of antipsychotics, especially low potency drugs such as chlorpromazine and thioridazine.1 Among atypical antipsychotics, clozapine has consistently been associated with VTE in young patients with psychiatric illnesses,2 but evidence from large observational studies has suggested that other atypical antipsychotics carry a similar risk, especially among new users and elderly patients.3 So far, however, the possibility that the underlying psychiatric disorders themselves—and not the antipsychotics—are associated with VTE has never been excluded. This could occur, for example, by the increased concentrations of adrenaline seen during psychotic excitation increasing blood coagulation.4

In the linked study (doi:10.1136/bmj.c4245), Parker and colleagues report a large population based case-control study that included primary care patients aged over 16 who were taking antipsychotics.5 In almost 99% of cases the reason for …