Lawyers are predicting massive numbers of lawsuits over Zantac, which drugmaker Sanofi recalled last week after the FDA discovered the heartburn medication contained an ingredient that could cause cancer.

"We'll see a real explosion of this litigation over the next four or five months," said Brent Wisner, who was in Las Vegas this week speaking about Zantac lawsuits at the Mass Torts Made Perfect conference.

So far, about a dozen lawsuits, both consumer class actions and individual personal cases, allege that Sanofi and other drug companies knew that ranitidine, the active ingredient in some heartburn medications, metabolizes into unsafe levels of a possible carcinogen, known as N-nitrosodimethylamine, or NDMA, but failed to disclose the information to their customers. The FDA earlier this month said its preliminary tests had discovered NDMA in certain ranitidine medicines, such as Zantac, but it is still investigating.

Wisner, of Los Angeles-based Baum Hedlund Aristei Goldman, predicted that the litigation could be huge, given that there are millions of customers of Zantac, which first sold in 1983. Four defendants are named in the lawsuits. His firm won a $289 million verdict last year and a $2 billion jury award this year on behalf of cancer victims suing over Monsanto Co.'s Roundup herbicide.

"This is [the] very, very, very beginning of this litigation," he said. "The mechanism through which this causes cancer is pretty much rock solid at this point, the science is pretty overwhelming, and frankly, it's a very widely used drug. So we're going to have potentially hundreds and hundreds and hundreds of thousands of people who qualify for a lawsuit. In that context, it will dwarf what we saw in Roundup."

The lawsuits follow the FDA's investigation of NDMA in some blood pressure and heart failure medicines. Last year, the FDA found a carcinogen in those medications, such as generic valsartan, prompting the filing of more than 140 lawsuits now coordinated in multidistrict litigation in New Jersey. Lawyers suing over Zantac sought to distinguish the Valsartan cases, which focus on an alleged manufacturing defect.

"This is not a manufacturing problem where NDMA got in the drug by accident," Wisner said. "This is a fundamental error with the way the chemical interacts with the human body."

On Sept. 13, an online pharmacy called Valisure filed a citizen petition demanding that the FDA pull ranitidine medications.

The FDA last month disclosed it was investigating the presence of NDMA in ranitidine medications but did not recommend that customers stop using them. Instead, drugmaker Novartis announced it was halting distribution of generic ranitidine medications made by its Sandoz division. Rite Aid, CVS, Walmart and Walgreens also voluntarily pulled ranitidine products, including Zantac, from store shelves.

On Oct. 2, the FDA found that preliminary tests showed "unacceptable levels" of NDMA in some ranitidine samples.

Then, on Oct. 8, GlaxoSmithKline recalled prescription Zantac and, on Oct. 18, Sanofi announced its voluntary recall of over-the-counter Zantac.

In a statement, Sanofi said the move was a "precautionary measure" due to "possible contamination."

"Evaluations are ongoing on both drug substance (active ingredient) and finished drug product," the company said. "Due to inconsistencies in preliminary test results of the active ingredient used in the U.S. and Canadian products, Sanofi has made the decision to conduct the voluntary recall in the U.S. and Canada as the investigation continues."

This week, another manufacturer, Dr. Reddy's Laboratories Ltd., voluntarily recalled its ranitidine medications, sold under its name and at retailers such as Kroger and Walgreens. The FDA said that preliminary testing indicated that other heartburn medications, such as Pepcid and Nexium, did not contain NDMA.

Wisner said that's because the other heartburn medications don't have the ingredient ranitidine.

He and firm managing partner Michael Baum, as well as partners Bijan Esfandiari and Nicole Maldonado, have filed three individual lawsuits in Pennsylvania, California and Colorado federal courts. Other firms have filed at least three separate injury lawsuits in Florida.

The suits seek punitive damages.

Wisner said the NDMA has ties to multiple cancers, including of the liver, bladder, kidney, prostrate, stomach, esophagus and colon. The plaintiffs in his firm's lawsuits are individuals diagnosed with kidney cancer, colorectal cancer and bladder cancer. The Florida lawsuits involve two women with breast cancer and a man with kidney cancer.

"There are 800,000 people diagnosed each year with the cancers we're talking about," he said. "How many are caused by ranitidine or Zantac? I'm not sure. But even if you're conservative, and just say 10%, you're looking at over 80,000 cancers a year caused by this drug."

So far, he said, more than 300 people have reached out to his firm, which has signed up at least 50 clients.

Seattle's Hagens Berman Sobol Shapiro, meanwhile, filed a case for a California class and nationwide class actions in federal courts in New Jersey and Connecticut, some which also contain subclasses in Massachusetts and Florida.

The suits, brought by managing partner Steve Berman and Chicago partner Jason Zweig, seek refunds for customers who purchased Zantac any time after Jan. 1, 2010.

"Millions of people in the U.S. suffer from heartburn, and for years, Zantac has been sold to the masses as a safe and easy-to-find remedy for that common ailment," Berman said in a statement. "We're certain that if those millions of consumers knew that the Zantac they take contains known carcinogens, they would be rightfully outraged."

Berman called the FDA a "shell of an agency" that has been more focused on protecting pharmaceutical companies than consumers. He said the FDA waited three months before disclosing any information about Zantac, and continues to misrepresent the problem.

"As we allege in our lawsuit, and we intend to prove, there is a fundamental defect with Zantac: when ingested, it forms a potent carcinogen (NDMA) in your body that exceeds FDA permissible intake limits by thousands of times," he said. "Instead of informing the public of this, the FDA is seeking to mischaracterize the situation with Zantac as one involving an 'impurity' and has asked the big pharmaceutical companies to perform tests that are intentionally designed to miss the key issue: that Zantac is an inherently dangerous drug. It is time someone speaks the truth about what is going on and we intend to speak that truth."

Bursor & Fisher also filed a nationwide class action in U.S. District Court for the Southern District of New York.

Representatives for Sanofi S.A., whose U.S. headquarters is in Bridgewater, New Jersey, and GlaxoSmithKline, with a U.S. unit based in Philadelphia, which first received FDA approval to sell the drugs, declined to comment. Other defendants are companies that later sold Zantac over the counter: New York-based Pfizer Inc. and Boehringer Ingelheim Pharmaceuticals Inc., with a U.S. headquarters in Ridgefield, Connecticut. Representatives from both companies did not respond to a request for comment.