Teva Pharmaceuticals has yet to buy Mylan Laboratories , but several advocacy groups are so leery a merger would harm consumers that they wrote a letter urging the U.S. Federal Trade Commission to block any deal that may occur between the generic drug rivals. They cite antitrust concerns that prices would rise, shortages would develop and some research and development would be curtailed.

Teva, you may recall, earlier this year made an unsolicited, $40 billion bid for Mylan, which promptly rejected the overture and did so in unusually harsh terms. In fact, Mylan executive chairman Robert Coury maintained that the unwanted deal would create antitrust risk and cause a combined company to unnecessarily divest valuable medicines.

Nonetheless, Teva has displayed persistence by brushing aside the barbs and warnings, and recently acquired 4.6% stake in its generic rival. Mylan, meanwhile, has continued its own pursuit of Perrigo , which is seen as a tactic to thwart the unwanted Teva bid. All of this jockeying has alarmed the consumer groups, since Teva and Mylan are the largest suppliers of generic drugs in the U.S.

In the past, the groups note the FTC has attempted to solve antitrust concerns by requiring a merged company to sell various drugs in order to lessen its dominance in different product categories. But in this instance, the consumer groups argue such a move is not likely to have the desired impact if only because a merger would create a behemoth.

“This merger is far bigger and more far reaching” than previous deals, the groups wrote. “A merged Teva/Mylan would have a 25% market share over all generic drugs, and would combine the largest number of overlapping, currently competing drugs of any generic drug company merger ever. The merged company would have a virtual lock on a number of key generics.”

A deal would create “a single entity with more than double the market share of the next-largest generic drug manufacturer,” the groups continue, noting that there is “significant overlap” between the 360 generic drugs made by Mylan and the 375 generic medicines made by Teva. “No divestiture of specific drugs would be sufficient to replace the loss of competition.”

The letter was sent by Consumers Union; Consumer Federation of America; U.S. PIRG; Public Citizen; Consumer Action; Consumer Watchdog; Community Catalyst and the National Center for Health Research.

A Teva spokesman writes us to say that “a combined Teva and Mylan would create an industry-leading and dependable supplier of the broadest portfolio of high quality medicine to consumers across global markets. This combination will create a more efficient platform for the benefit of customers and consumers.

“Teva remains deeply committed to consummating a transaction with Mylan that offers an unparalleled opportunity for greater access to much needed medicines… Teva is confident it can quickly make any necessary divestitures to achieve antitrust clearance. Teva filed for U.S. antitrust clearance several months ago and has initiated the pre-merger notification process with the European Commission.”

A Mylan spokeswoman sent us this: The letter “further shines a spotlight on how the market dominance of a combined Teva/Mylan could significantly reduce competition, resulting in higher prices, delayed entry of generics and diminished innovation. This transaction also would serve to eliminate the only major generics company that is not driven by significant branded business interests, thereby silencing one of the strongest advocates for the generics industry.

“As we have said all along, we do not believe that these concerns can be remedied through product divestitures, and we believe that the FTC and other regulators will take all of these factors into consideration in their analysis. Certainly, this deal faces a long and difficult review by the antitrust regulators and we believe it will ultimately be rejected.”

As for the FTC, a spokeswoman says she was unable to confirm that the agency has received the letter, but that “we’ll give it serious consideration, as we do for all letters” expressing antitrust concerns.