Wyeth has received a boost in the US courts after a judge in Philadelphia declared a mistrial for a case from last week when a jury ruled that the use of the firm¹s hormone replacement therapy Prempro caused breast cancer to develop in a 67-year-old woman who had used the treatment for six years.

Jennie Nelson was originally awarded $1 million and her husband $500,000 in damages. The jury was then supposed have moved onto a second phase of the trial to determine if Wyeth was actually liable for those damages but judge Norman Ackerman granted the company's motion for a mistrial, a move which was unopposed by plaintiffs.

No details as to why the motion was unopposed were given and Mrs Nelson’s lawyers limited themselves to telling the press that it was a result of “extraneous circumstances.”

The mistrial ruling follows the first federal Prempro ((conjugated estrogens/medroxyprogesterone acetate) trial, where a jury in Little Rock, Arkansas concluded that Wyeth was not negligent and had adequately warned patients and doctors of the cancer risk associated with the drug.

As many as six million women were thought to be taking Prempro before results from a Women’s Health Initiative study in 2002 linked the drug to a significant increase in risk for invasive breast cancer, coronary heart disease and stroke. Wyeth is currently facing more than 5,000 lawsuits involving its HRT products.