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KAI RYSSDAL: Merck has been the drugmaker in the news the past couple of months. It's fighting off thousands of Vioxx lawsuits one by one. But another company is set to steal that unwelcome spotlight. Wyeth is defending about 4,500 suits over its hormone replacement, Prempro. Six million menopausal women in this country have taken it. But the drug has been linked to breast cancer. Jury selection in the first case to go to trial starts tomorrow. Helen Palmer reports from the Marketplace Health Desk at WGBH it's not Wyeth's first time in court.

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HELEN PALMER: For the last nine years Wyeth's been struggling with litigation over Fen-Phen, the diet drug combo found to cause heart damage. Nearly 200,000 women have filed claims, and settlements and judgments cost the company over $21 billiion. But analysts say the Prempro litigation's not likley to be nearly as damaging.

IAN SANDERSON: It's going to be very difficult for plaintiffs to prove causality here, that Prempro actually caused their breast cancer.

Cowen and Company pharma analyst Ian Sanderson. He says the actual number of extra breast cancer cases is small: eight extra women in 10,000 who took the hormones every year. And Sanderson says the drug's label warns of the risk, giving the company a solid defense.

Attorney Mike Doyle, who usually represents plaintiffs, agrees. Wyeth's got a strong case because the FDA didn't pull the drug off the market.

MIKE DOYLE: But there's two big things they can point to — with it not being off the market and some recognized health benefits.

The plaintiff's may have some powerful ammunition too. Doyle says there's evidence that sales reps and marketing documents knowingly played down the drug's dangers.

DOYLE: Those are powerful, compelling documents that make it hard to justify the massive marketing campaign where they hired Lauren Hutton to say everybody ought to use this because it's such a great product.

Wyeth shelled out nearly $35 million to push Prempro in 2000. Cowen's Ian Sanderson says Wall Street's not too fussed about the litigation.

SANDERSON: The economic value of Prempro took the major hit back in 2002 when these results were initally released. I think the market will shrug this off.

That's unless, of course, the case should go against the company and the jury slap it with a large liability judgment.

In Boston, I'm Helen Palmer for Marketplace.