It's about allegations that the osteoporosis drug resulted in injury to jawbone tissue.
The trial in New York of the first case of the group, filed by Shirley Boles, 71, will be one of three so-called bellwether cases that may point the way to out-of-court settlements.
"In mass litigation, all eyes are on the first trial, not only because it shows the strategy of each side, but also because it's the first information about how jurors respond to the evidence," said Howard Erichson, a law professor at Fordham University in New York and an expert on civil procedure.
There are as many as 1,200 plaintiffs in state and federal cases, Merck says. Timothy O'Brien, a lawyer representing about 400 plaintiffs, said the number is closer to 2,000.