US giant Wyeth, which has only recently turned the corner on a multi-billion-dollar lawsuit for the diet drugs fenfluramine and dexfenfluramine, will this week find itself back in court again over allegations that its menopause therapies can cause breast cancer.

The first trial is due to begin today in Little Rock, Arkansas, with plaintiff Linda Reeves - who is 67 - claiming her long-term use of Wyeth's combined hormone product Prempro to prevent osteoporosis led to breast cancer and a double mastectomy. Attorneys involved in the more than 5,000 cases lined up against Wyeth have accused the firm of misleading patients and healthcare professionals about the risks of hormone replacement therapy, and over-promoting it as what one of the lawyers has nicknamed “a fountain of youth.”

Wyeth's stock plunged by more than a quarter in 2002 after a major US government-led study found combined oestrogen and progestin (Prempro in this case) was associated with an increased risk of breast cancer, coronary heart disease and stroke. The study, which was due to run until 2005, was terminated at an average follow up of 5.2 years after investigators observed a 26% increase in breast cancer, a 41% rise in strokes and a 29% increase in heart attacks.

The findings sent waves across the world, with the market for HRT products plunging dramatically as regulators moved quickly to slap warnings on their use. But Wyeth has been boosted of late after findings from the same Women's Health Initiative study revealed that a different product - oestrogen-only replacement therapy - is not associated with an increased risk of breast cancer in post-menopausal women, and may even give some protection against the disease. It has also been shown recently to cut the risk of heart disease in older women.

But such positive news for oestrogen-only therapy does not help Wyeth's current situation. Having seen settlement and legal costs from its phen-fen litigation hurdle over the $20 billion mark, and having witnessed the scale of the litigation against Merck & Co for its now withdrawn painkiller Vioxx (rofecoxib), it may be feeling a little nervous at what the prospects of this latest round of cases may hold. Prempro is, however, still on the market - helping generate $260 million from the firm's HRT family of products in the second quarter - so, although the findings of the WHI trial are indisputable, Wyeth is likely hoping this will prove to be a clear differentiator.