NEW YORK (CNNMoney.com) -- Wyeth faces another lawsuit Wednesday - with thousands more to go -blaming its drug Prempro for causing breast cancer.

Jury selection has begun in Pennsylvania state court in Philadelphia for a lawsuit filed by plaintiff Jenny Nelson, 61, a retired secretary and grandmother who says her five years of taking Prempro to treat her menopause caused the cancer that led to her double mastectomy.

This is the second case filed against Wyeth, the sixth-largest drugmaker in America, and opening arguments are scheduled to begin on Sept. 11. The first case, filed in federal district court in Little Rock, Ark., by 67-year-old plaintiff Linda Reeves, is ongoing.

The company faces at least 4,000 more cases from former patients of Prempro, whose sales totaled $1 billion in 2005.

The 4,000 or so Prempro cases are coming to trial as memories of fen-phen - the diet drug cocktail that Wyeth pulled off the market in 1997 after it was linked to heart valve disease - are still fresh. In 1999, the New Jersey-based drugmaker agreed to pay a $3.75 billion settlement to former fen-phen patients.

Investors, though, seem untroubled by the serious accusations against its menopausal drug, taking comfort in the company's solid pipeline and few patent expirations. Wyeth's stock price has risen more than 5 percent so far this year, outperforming the flat S&P 500.

Lawsuits against drug companies have become a ubiquitous part of the business, and they have a way of shaking investor confidence. The stock price for Merck, another New Jersey-based pharmaceutical firm and the no. 4 U.S. drugmaker, has plunged 19 percent since Sept. 30, 2004, when the company pulled its blockbuster drug Vioxx off the market because a study demonstrated increased risk of heart attacks.

But the Prempro lawsuits have failed to shake investor resolve.

"I don't get the sense that anyone's that concerned about it, primarily because Prempro is still on the market," said Jon LeCroy, analyst for Natexis Bleichroeder.

LeCroy said that Prempro's presence on the market "shows that the FDA believes that they're safe enough for the market" and suggests that the drug's links to breast cancer are not "clear cut or severe."

(An earlier version of the story misstated how Wyeth stock has performed during the reign of CEO Robert Essner. CNNMoney.com regrets the error.)

Wyeth faces thousands of Prempro lawsuits