Singulair (Montelukast) Lawsuit 2026: FDA Boxed Warning, Neuropsychiatric Injuries, and Where Litigation Stands

Montelukast is one of the most widely prescribed drugs in the history of American medicine. At its peak, Singulair — the brand-name version — was among the top-selling prescription medications in the world. Millions of Americans, including young children, took it for years to manage asthma and allergic rhinitis.

The 2020 FDA boxed warning changed the legal landscape fundamentally. It acknowledged that serious psychiatric effects — suicidal ideation, depression, aggression — were real, clinically significant, and not adequately communicated to prescribers and patients. That acknowledgment is the foundation of the ongoing litigation.

What Montelukast Is and Why It Was So Widely Prescribed

Montelukast is a leukotriene receptor antagonist (LTRA). It works by blocking leukotrienes — compounds released in allergic reactions that cause inflammation and bronchoconstriction. By blocking this pathway, it reduces symptoms of asthma and allergic rhinitis without the immediate side effects of corticosteroids.

Merck brought it to market under the Singulair brand and it became a standard first-line treatment. Critically, it was approved for children as young as 12 months for asthma. Its ease of use — once-daily oral dosing, available in chewable tablet and granule formulations — made it the prescription of choice for pediatric allergists and pulmonologists. That pediatric reach is a significant factor in the litigation.

After Merck’s patent expired, generic versions flooded the market and prescribing volume remained high. The defendants in this litigation include Merck (the brand manufacturer) and generic manufacturers.

The 2020 FDA Boxed Warning: What It Said and Why It Matters

The FDA has been aware of potential neuropsychiatric effects of montelukast since at least 2008, when it first required label changes acknowledging the possibility. But the 2008 label update was a standard warning, not a boxed warning.

In March 2020, the FDA escalated to a boxed warning for all montelukast products. The FDA’s stated rationale was direct: “many health care professionals, patients and caregivers were not aware of the risk” of serious neuropsychiatric effects.

The warning covers these events:

• Suicidal thoughts and behavior
• Depression
• Aggression
• Agitation and anxiety
• Sleep disturbances (including insomnia and vivid dreams)
• Hallucinations
• Tremors
• Memory problems
• Nightmares

The FDA also changed its guidance on prescribing for allergic rhinitis specifically. For that indication, where other effective alternatives with better-understood risk profiles exist, the FDA recommended that prescribers “generally avoid” montelukast unless the patient previously tried and failed other options. The implication was clear: the risk-benefit calculus for allergic rhinitis was no longer as favorable.

Context on pharmaceutical labeling and failure-to-warn claims →

Children at the Center of the Litigation

FDA adverse event reporting data through March 2024 recorded 7,944 total psychiatric event reports associated with montelukast. Of those, 2,872 involved children between 2 months and 17 years of age — roughly 36% of all reports.

This is not just a statistic. It reflects the clinical reality that montelukast was aggressively prescribed to young children for years, often for extended durations. Parents whose children developed sudden behavioral changes — aggression, self-harm, severe sleep disruption, suicidal ideation — while on montelukast frequently did not know the medication might be responsible. Some families watched their children struggle for years before making the connection.

For plaintiffs’ attorneys, the pediatric cases are often the most compelling. A child who developed suicidal ideation or required psychiatric hospitalization while taking montelukast for asthma — and whose symptoms resolved after the medication was stopped — presents a strong temporal causation narrative.

MCL 637: Where the Litigation Lives

The Singulair litigation is consolidated in New Jersey Superior Court, Multicounty Litigation No. 637 (MCL 637). MCL proceedings are New Jersey’s state-court equivalent of a federal MDL — they centralize discovery and pretrial management for similar claims.

New Jersey was the natural venue because Merck’s principal U.S. operations and registered offices are in New Jersey.

Key litigation developments as of 2026:

• In May 2025, a settlement agreement was reached for a group of cases within the MCL 637 proceeding. Specific settlement terms and individual compensation amounts were not publicly disclosed.
• A separate reported development: FDA researchers reportedly determined that montelukast attaches to brain receptors critical to psychiatric functioning, though findings have not been fully released publicly.
• Litigation continues for cases not resolved in the 2025 settlement.

Merck’s Defense Strategy

Merck’s response to these lawsuits rests on several overlapping arguments.

First, prior disclosure: Merck argues that neuropsychiatric effects were disclosed in the labeling before the 2020 boxed warning, in the post-market safety updates following the 2008 review. The boxed warning, they contend, represents an escalation in emphasis rather than the revelation of a previously concealed risk.

Second, causation challenge: Patients who are prescribed montelukast have asthma or allergies — conditions that may independently contribute to anxiety, depression, or sleep problems. Disentangling drug-caused psychiatric effects from condition-caused psychiatric effects is a genuine scientific challenge. Merck will use this uncertainty aggressively in individual case defense.

Third, learned intermediary doctrine: Merck may argue that adequate warnings were given to prescribing physicians, and physicians are responsible for communicating risks to patients. This doctrine limits manufacturer liability in many states when the physician is adequately warned.

Each of these defenses is contestable, and plaintiffs have strong counterarguments — particularly regarding the sufficiency of warnings prior to 2020 and the FDA’s own finding that awareness of the risk was inadequate.

Who Has a Viable Claim

There is no bright-line test, but the factors that tend to produce stronger cases are:

Factor	Significance
Duration of use	Longer use strengthens the temporal case
Severity of event	Suicide attempt, hospitalization > sleep disturbance
Temporal correlation	Symptom onset during use; improvement after stopping
Age at use	Pediatric cases are often stronger given the FDA data
Alternative cause exclusion	Ruling out independent psychiatric conditions helps

An attorney evaluating a Singulair case will want to see prescription records, medical records documenting the psychiatric event, records showing the timeline of drug use relative to symptom onset, and documentation of what happened after the medication was stopped.

Understanding structured settlements and how lump-sum versus periodic payments work →

Hernia mesh litigation as another example of medical product failure-to-warn cases →

What to Do if You Believe You Have a Claim

1. Gather prescription records — Document exactly when you (or your child) started and stopped montelukast, the prescribing physician, and the dosage.
2. Document the psychiatric event — Emergency room records, psychiatric hospitalization records, therapy records, and any physician notes discussing behavioral or mood changes.
3. Note what happened when the medication stopped — Resolution of symptoms after discontinuation is a significant factor in the causal narrative.
4. Check your state’s statute of limitations — Most states provide two to four years from the date of injury discovery, but child plaintiff rules vary. Consult an attorney sooner rather than later.
5. Understand that initial consultations are free — MCL 637 cases are handled on contingency. You pay nothing upfront.

The 2020 boxed warning is the central fact in this litigation. It means the FDA itself determined the risk was serious and that earlier communications were inadequate. That determination does not automatically establish Merck’s liability in any individual case, but it significantly shapes how a jury views the question of whether patients were warned appropriately.

This post is for informational purposes only and is not legal advice. Consult a licensed attorney in your state for guidance specific to your situation.