Switching patients from Pfizer’s cholesterol blockbuster Lipitor to a generic version of Merck & Co's Zocor has been linked to a major increase in the risk of serious heart problems, according to a new study sponsored by the former firm.

Pfizer says that the observational study of a large UK primary care database showed that switching patients from Lipitor (atorvastatin) to simvastatin was associated with a 30% increase in the relative risk of major cardiovascular events, including heart attacks and strokes. The analysis was presented at the European Society of Cardiology meeting in Vienna, Austria, and will be published in The British Journal of Cardiology.

The data, which included records from October 1997 to June 2005, were generated from an analysis of a medical database of anonymous patient records in the UK called The Health Improvement Network (THIN) and included 11,520 patients. Of those, 2,511 had taken Lipitor for six months or more and were switched to simvastatin versus 9,009 patients who were on the Pfizer drug for the same length of time but remained on Lipitor therapy.

Apart from the 30% rise in the risk of death, heart attack, stroke and heart surgery, a secondary analysis of the same data showed that patients who were switched to simvastatin were more than twice as likely to discontinue their treatment compared to those who remained on Lipitor (20.5% versus 7.62%). The reasons for discontinuation were not available from the database, though disruption in treatment has been associated with poor adherence in previous studies of statins and other medications.

Michael Berelowitz, senior vice president of Pfizer’s global medical division, said that nowadays governments and managed care companies are encouraging patients who are well-established on one therapy to switch to a different statin but “this study raises concerns about those policies”. He argued that the results of this analysis “complement the large body of evidence from multiple clinical trials demonstrating the cardiovascular benefits of Lipitor” and highlights the need to “carefully consider individual patient circumstances and cardiovascular risk because indiscriminate switching may adversely affect some patients”.

The launch of generic Zocor, which lost patent protection in the USA June last year, has hurt Lipitor as health insurers have been encouraging patients to switch to the cheaper statin. In the second quarter of 2007, sales of Lipitor in the USA stumbled 25% to $2.7 billion.