Now,

Pfizer

(PFE) - Get Report

is causing some hearts to pound.

The giant drugmaker on Friday warned that its popular painkiller Celebrex has been linked to heart attacks in a new study. The announcement comes less than three months after rival

Merck

(MRK) - Get Report

withdrew a similar blockbuster, Vioxx, because of cardiac risks.

At the time of the Vioxx recall, Pfizer expressed confidence in the safety of its own Celebrex and even recommended the drug as "an appropriate treatment alternative" for Vioxx users seeking new medication. Jake Dollarhide, CEO of Longbow Asset Management in Tulsa, Okla., remembers cringing at the "foolish" comments by the company.

"Pfizer walked a very dangerous tightrope when it proclaimed, just after Merck's shocking announcement about the Vioxx withdrawal, that Celebrex had a clean bill of health," said Dollarhide, whose clients own Pfizer shares. "I hoped those words would not come back to haunt them."

They did.

Pfizer's stock plunged 14% to $24.97 -- a new 52-week low -- on Friday's news. Merck suffered sympathy pains, falling 3.8% to $30.57.

Despite concerns about Vioxx-like painkillers -- which, by design, can reportedly cause blood clotting -- Pfizer has consistently portrayed Celebrex as safe. But one of two long-term studies has now shown a 2.5-fold increase in heart attacks among patients using high doses of the drug. Pfizer CEO Hank McKinnell described the findings as both "unexpected" and "not consistent" with other results, including those released this week that showed no increased cardiac risk among patients on normal doses of the drug.

"Pfizer is taking immediate steps to fully understand the results," McKinnell said, "and rapidly communicate new information to regulators, physicians and patients around the world."

Pfizer is not, however, following in Merck's footsteps by yanking the drug from the market. At least one attorney, who is representing thousands of Vioxx plaintiffs, believes the company should.

William Federman, a class-action attorney in Oklahoma City, called Pfizer's position an "aggressive" one that could bring legal risks. But for now, he said, Pfizer probably still believes that risk is worth taking.

"It's just basic economics," he concluded.

Celebrex, a multibillion-dollar blockbuster for Pfizer, ranks as the most prescribed arthritis pain-relief drug in the world. But some have already started imagining Pfizer without the popular painkiller.

Under a worst-case scenario, Goldman Sachs analyst James Kelly believes that Pfizer's earnings could drop enough to wipe $5.25 from Friday's opening stock price of $28.98 a share. The stock was down $4.10 halfway through the session.

"The market's reaction to this news suggests that investors are assuming that Celebrex and Bextra (a similar painkiller) will be pulled from the market, which may be too pessimistic a view," wrote Morgan Stanley analyst Jami Rubin. "However, it does not appear that investors are assuming any significant legal exposure" for the company.

But Dollarhide, for one, can see the lawsuits coming -- especially if Pfizer knew about Celebrex's cardiac risks but proclaimed the drug safe anyway. Critics have already accused Merck of covering up dangers associated with its own Vioxx.

Both Celebrex and Vioxx, along with Pfizer's Bextra, belong to a new class of painkillers known as COX-2 inhibitors that hit the market with considerable fanfare five years ago. The drugs supposedly offer pain relief without the same stomach bleeding caused by cheaper, over-the-counter alternatives like ibuprofen. But critics have questioned both the effectiveness and the safety of the COX-2 drugs almost from the beginning.

Public Citizen, a nonprofit consumer watchdog organization, began placing all of the COX-2 drugs on its "worst pills" list years ago. The group officially declared the so-called super aspirins "super disasters" after Merck's recall of Vioxx in September.

"Although Merck's withdrawal of Vioxx 'solves' the safety problems with this drug," Public Citizen wrote, "the most-prescribed alternatives -- Celebrex and Bextra -- also have some concerns about their cardiac toxicity."

Over the years, Public Citizen has criticized both Merck and Vioxx for allegedly downplaying the risks associated with their COX-2 inhibitors. It noted that both had fielded warnings from the Food and Drug Administration about their promotion of the painkillers.

Many, including Dollarhide, believe that heavy promotion drove consumers to ask their doctors for the drugs by name and, therefore, fuel their popularity. But Dollarhide now wonders whether doctors will be as willing to write prescriptions for COX-2 inhibitors -- even if they do stay on the market.

He believes the potential cardiac risks may simply look too great.

"We're talking about a drug for arthritis here; we're talking about joint pain," he said. "That's not a very good trade-off."

Still, Dollarhide plans to keep Pfizer -- with its large portfolio of blockbuster drugs -- as a holding unless he discovers some kind of "untreatable cancer within the company." Specifically, he says he would drop the stock "if it turns out that Pfizer made some of the same, or even less ethical, moves" that Merck has been accused of making.

To be fair, both companies have stood by their actions. But Dollarhide foresees big legal bills if they have knowingly done anything wrong. By now, Merck already faces a slew a patient lawsuits. But Dollarhide worries that Pfizer -- with its deeper pockets -- could face even bigger risks if troubling evidence about its own COX-2 inhibitors starts to surface as well.

"Would you rather sue the guy who is driving the Honda Accord or the guy who is driving the Mercedes-Benz?" he asked. "Personally, I think you might sue both."