The drugmaker has sent a “small number” of letters demanding lawyers pull ads making false or misleading claims about links between its diabetes med and heart attacks. “Lawyers have a right to advertise, but they have to play by the rules just like we do,” Chris Viehbacher, president of Glaxo’s U.S. unit, tells Bloomberg News. Since letters have gone out, Glaxo has “seen a dramatic decline” in TV advertising seeking Avandia cases.

The letter sent by the Philadelphia law firm Pepper Hamilton contends that the plaintiffs firm’s ads “are actionable” because they imbue the study with “a definitive, conclusory quality.” A recent study in The New England Journal of Medicine found Avandia users were 43 percent more likely to have a heart attack than those taking other diabetes meds.

“They think by threatening lawyers, they can shut the courthouse doors,” Mark Lanier, a Texas lawyer who won the first Vioxx case against Merck. David Ratner, a New York lawyer, tells Bloomberg that he pulled TV ads after receiving the letter, but continues to advertise for cases on the Internet. “We got about 12,000 calls and about 1,200 potential cases out of them.”

Howard Erichson, a Seton Hall University law professor who teaches product liability law, says that lawyers whose advertising includes false, deceptive or misleading claims can be sued. Glaxo is “trying to kill two birds with one stone” with its letter campaign, he says. “They want to back the lawyers off from advertising aggressively while defending the reputation of this drug.”