AstraZeneca

- Modern antipsychotic drugs, especially AstraZeneca's blockbuster Seroquel, may increase the risk of patients developing life-threatening blood clots, British researchers said on Wednesday.

The finding, from a trawl of tens of thousands of patient records, adds to a growing body of evidence linking so-called atypical antipsychotics to adverse side effects.

Atypicals -- which also include Eli Lilly's Zyprexa, Johnson & Johnson's Risperdal and Bristol-Myers Squibb's Abilify -- dominate the antipsychotic market, which had sales of $16 billion (10 billion pounds) in 2009, according to Thomson Reuters data.

First introduced more than 20 years ago, such drugs cause fewer of the involuntary-movement problems associated with older medicines but have been linked to diabetes and increased mortality in people with dementia.

There has also been limited evidence linking them with dangerous blood clots, such as deep vein thrombosis and pulmonary embolism, although previous studies have been small.

The large British study, which compared 25,000 cases of patients with venous thromboembolism (VTE) against 89,000 controls, found people prescribed antipsychotics had a 32 percent greater risk of serious blood clots, after adjusting for other potential risk factors.

The risk was greater for people on atypical drugs, rather than older ones, and the highest risk was seen with Seroquel, which showed a nearly three times increase on an adjusted basis.

However, the researchers pointed out that the absolute risks still remained low, with an excess of four extra cases of VTE per 10,000 patients across all antipsychotics, and 21 in the case of Seroquel.

"Though these findings add to the accumulating evidence of adverse health events associated with antipsychotic drugs, they should be confirmed with other data sources," Julia Hippisley-Cox from Nottingham University and colleagues wrote in the British Medical Journal.

Rosa Liperoti and Giovanni Gambassi of Rome's Universita Cattolica del Sacro Cuore said in a separate comment that the findings showed the need to identify the best candidates for antipsychotic treatment and work out those susceptible to developing side effects.

AstraZeneca said it would review the latest study results but the company stood by the drug's risk-to-benefit profile. A spokeswoman noted that VTE was already mentioned in the product's label as a rare adverse drug reaction.

The company agreed last month to pay $198 million to settle some 17,500 U.S. personal injury claims related to Seroquel, following legal claims about its link to an increased risk of diabetes.