Causation in Complex Torts:  

Lessons from Zantac and Paraquat Dismissals 

In complex tort litigation, particularly toxic torts and mass actions, proving causation is complicated by scientific uncertainty, multiple potential causes, and long latency periods between exposure and injury—challenges expressly recognized in the Report on Mass Torts by the Judicial Conference of the United States.¹  

The Zantac multidistrict litigation (MDL No. 2924) illustrates these difficulties. In December 2022, Judge Robin L. Rosenberg excluded all of plaintiffs’ general-causation experts under Daubert, and entered summary judgment for defendants, holding that the methodologies offered were unreliable and not grounded in generally accepted science. (In re Zantac (Ranitidine) Prods. Liab. Litig., 644 F. Supp. 3d 1043, 1052–53 (S.D. Fla. 2022) Similarly, in the Paraquat MDL (MDL No. 3004, S.D. Ill.), Chief Judge Nancy J. Rosenstengel excluded plaintiffs’ key expert on general causation in April 2024, finding his methodology “unscientific cherry-picking” and lacking peer-reviewed validation, which led to dismissal of the first bellwether cases (In re Paraquat Prods. Liab. Litig., No. 3:21-MD-3004-NJR, Doc. 3738 (S.D. Ill. Apr. 5, 2024))². Together, these rulings demonstrate how the gatekeeping role of federal courts under Daubert can be outcome-determinative in complex torts, with the admissibility of scientific evidence effectively dictating whether mass claims survive. 

This gatekeeping function is critical in complex torts, where the admissibility of expert testimony often determines the case’s outcome. 

Two massive pharmaceutical lawsuits collapsed in spectacular fashion over the past two years, sending shockwaves through the mass tort bar and reshaping how courts evaluate scientific evidence in complex litigation. The dismissals of thousands of Zantac cancer claims and Paraquat Parkinson’s disease lawsuits demonstrate how rigorous application of scientific standards can derail even the largest multidistrict litigations, leaving plaintiffs empty-handed despite years of litigation and millions in legal expenses. 

The Zantac multidistrict litigation appeared to be plaintiff attorney’s dream case, with over 70,000 consolidated claims alleging the popular heartburn medication caused cancer.³ Then came December 2022, when Judge Robin L. Rosenberg excluded every single plaintiffs’ causation expert under Daubert, finding their methodologies unreliable and not grounded in accepted science, effectively ending the entire litigation overnight.⁴  

Similarly, the Paraquat litigation seemed more scientifically grounded, with epidemiological studies suggesting links between the herbicide and Parkinson’s disease. However, in April 2024, Chief Judge Nancy J. Rosenstengel excluded the key general-causation expert for “unscientific cherry-picking,” leading to dismissal of the first bellwether cases and signaling the end of the broader litigation.⁵  

These rulings mark a watershed moment in mass tort practice, demonstrating that even cases with thousands of plaintiffs and billions in potential damages can collapse entirely when the underlying scientific evidence fails to meet heightened judicial scrutiny under Daubert.  

I. Zantac Case 

Updates on Zantac Lawsuit– September 2025 

The Zantac litigation has been highly complex and dynamic, involving thousands of claimants alleging cancer caused by NDMA contamination in ranitidine products, most notably the brand-name drug Zantac. The operative plaintiff allegations are that Zantac degraded into NDMA, a probable human carcinogen, after manufacture and sale, causing various cancers.  

Major Settlement and Litigation Status 

• $2.2 Billion Settlement: In mid-2025, GlaxoSmithKline (GSK) reached a landmark settlement of up to $2.2 billion to resolve approximately 80,000 lawsuits predominantly filed in Delaware Superior Court. This settlement covers the bulk of claims against GSK and marks the largest resolution to date in the Zantac litigation.⁶ 

• Decline of Federal MDL Cases: After aggressive Daubert rulings in federal court excluded plaintiff-specific expert causation testimony, most federal Zantac lawsuits were dismissed. By 2025, only about 2,400 federal MDL cases remained, many poised for settlement or transfer to state courts.⁷ 

• Active Litigation Against Other Defendants: While GSK has settled most claims, litigation against Boehringer Ingelheim, Sanofi, Pfizer, and generic manufacturers continues, some of which remain active in state courts such as Delaware, California, Illinois, and Pennsylvania. 

II. Paraquat Case 

In re Paraquat Products Liability Litigation (MDL No. 3004, S.D. Ill.), Judge Nancy Rosenstengel dismissed four bellwether cases in April 2024 after excluding the testimony of the plaintiffs’ key expert, Dr. Martin Wells on general causation. The judge deemed his methodology unreliable, citing “unscientific cherry-picking” and lack of peer-reviewed validation outside the litigation. This decision left plaintiffs unable to establish the necessary causal link. Despite these setbacks, the litigation continues, with new bellweather cases selected for discovery and plaintiffs working with additional experts to strengthen their case. 

In re Paraquat Products Liability Litigation, highlights the difficulties of proving causation in cases involving environmental toxins, where exposure can be widespread and the latency period between exposure and disease onset is long. The exclusion of Dr. Wells’ testimony in both federal and state courts underscores the high bar for admitting scientific evidence in mass tort litigation.⁸ Despite setbacks, the MDL remains active.  

On August 6, 2024, the Court selected ten new cases for case-specific discovery, and on October 31, 2024, it issued an order replacing two voluntarily dismissed bellwether cases with new plaintiffs, ensuring the litigation’s momentum continues.⁹ 

Paraquat Lawsuit Updates – September 2025 

Settlement Progress and Status 

The Paraquat litigation continues to see major developments in 2025. Most notably, negotiations on a significant settlement for thousands of plaintiffs remain underway. In April, parties announced a tentative agreement intended to resolve a large number of claims in the multidistrict litigation (MDL) against manufacturers Syngenta and Chevron. This means a stay has been placed on active MDL litigation, giving both sides time to finalize the terms. 

Judge Nancy Rosenstengel, who oversees the Southern District of Illinois MDL, has now extended this pause until at least September 26, 2025, while parties hammer out remaining details. Although the framework for a deal is largely set, progress has slowed due to objections from certain plaintiff firms. If no final settlement is reached by late September, discovery and bellwether trial activities will resume—with a first trial still nominally set for October 14, 2025.¹⁰ 

Case Numbers and State Court Filings 

• Over 6,300 Paraquat claims are consolidated in the federal MDL, with more lawsuits still being filed in state courts nationwide.¹¹ 

• New filings and proceedings in state courts remain active as not all plaintiffs are included (or satisfied) with the proposed federal MDL settlement. 

Law and Policy Developments 

• Congressional interest is high: Senator Cory Booker’s “Pesticide Injury Accountability Act”—which would make it easier for those harmed by pesticides to access federal courts—continues to gain political and public support. 

• The Paraquat litigation highlights ongoing scrutiny of pesticide safety regulations and the adequacy of manufacturer warnings.¹² 

The Zantac and Paraquat litigations highlight the formidable challenges of proving causation in complex tort cases. Both cases underscore the critical role of reliable scientific evidence and expert testimony, as well as the stringent standards courts apply under Daubert. The federal MDL dismissals in both litigations demonstrate the high bar for plaintiffs, particularly when scientific consensus is lacking. However, ongoing state court actions and new bellwether cases suggest that plaintiffs are adapting their strategies to overcome these hurdles. 

These cases will likely shape future tort litigation by emphasizing the need for plaintiffs to present scientifically sound evidence and for defendants to leverage Daubert challenges effectively. As the legal landscape evolves, the lessons from Zantac and Paraquat will guide how causation is addressed in complex torts, ensuring that justice balances scientific rigor with fairness to all parties.