Taking antipsychotics could increase by nearly a third the risk of developing dangerous blood clots, an observational study published in the BMJ has found.

The case-control study of primary care patients in the UK by researchers from Nottinghamshire County Teaching Primary Care Trust and the University of Nottingham showed that patients prescribed antipsychotics over the previous 24 months had a 32% higher risk of venous thromboembolism – an umbrella term for deep vein thrombosis and pulmonary embolism – than non-users, even after adjusting for potential risk factors.

The risk was around twice as high for patients who had started taking a new antipsychotic in the previous three months. It was also more pronounced for patients given atypical rather than conventional antipsychotics (adjusted odds ratio of 1.73 versus 1.28) and for those prescribed low- rather than high-potency drugs (adjusted odds ratio of 1.99 versus 1.28).

The absolute risk of venous thromboembolism associated with use of antipsychotics was relatively low, estimated at an extra four cases over one year per 10,000 patients treated in patients of all ages, and an extra 10 cases per year in patients aged 65 years and over.

Nonetheless, the findings could have important implications for general practice, the researchers suggested, particularly as nearly all of the antipsychotics in the primary care population studied were prescribed for conditions such as nausea, vomiting and vertigo.

The reputation of antipsychotics has previously been dented by associations with over-use and increased risks of mortality and stroke in people with dementia, as well as with other conditions such as diabetes.

As the researchers led by Professor Julia Hippisley-Cox of Nottingham University’s Division of Primary Care pointed out, other case-control or cohort studies have suggested a link between antipsychotics and venous thromboembolism. However, their conclusions were muddied by the small numbers of patients involved or other factors such as exclusion of older people, limitation to patients aged 65 and over, or large disparities in effect between atypical and conventional antipsychotics.

The team led by Hippisley-Cox used the UK QResearch primary care database to examine the cases of 25,532 eligible patients aged 16 to 100 years with a first ever record of venous thromoboembolism (15,975 cases of deep vein thrombosis, 9,557 of pulmonary embolism) between 1 January 1996 and 1 July 2007.

The study’s findings would have to be replicated on another database before changes in clinical practice could be recommended, and larger numbers would be needed to estimate the risks associated with individual antipsychotics, the researchers noted.

If the findings were confirmed by other studies, though, antipsychotics should be used “more cautiously” for conditions such as nausea and agitation, especially in patients at high risk of thromboembolism, they said.