The U.S. Food and Drug Administration approved Tavneos in 2021 to treat rare autoimmune diseases, but a mandated postmarket safety study remains significantly delayed. According to FDA records, the study was to enroll 300 patients to monitor long-term safety, yet only 21 had been enrolled by fall 2025, the agency reported in an April letter. The delay highlights a broader issue: hundreds of similar studies required by the FDA are behind schedule, leaving patients and doctors without critical safety data on medications generating billions in revenue.
Delayed Safety Studies Raise Concerns
The FDA frequently approves drugs with provisos requiring manufacturers to conduct additional studies after market entry. These postmarket studies aim to address safety gaps or confirm effectiveness in larger, more diverse populations. However, a KFF Health News analysis of FDA data found nearly 600 such studies delayed, affecting almost 350 products. In some cases, delays span over a decade, as manufacturers prioritize profits over completing the mandated research.
Trump-era policies aimed at accelerating FDA drug reviews have intensified reliance on follow-up research. In February 2025, the agency announced that future approvals would require only one clinical trial instead of two, shifting more scrutiny to postapproval studies. Sanket Dhruva.
Medicare and Medicaid spent over $18 billion from 2018 to 2021 on accelerated-approval drugs with incomplete confirmatory trials, according to the HHS Office of Inspector General. While companies can continue selling products during study delays, patients may unknowingly use treatments with unresolved risks. For Tavneos, the FDA has identified dozens of cases of liver injury “possibly” or “probably” linked to the drug, one of the side effects the delayed study was meant to evaluate.
Manufacturers Cite Challenges, FDA Disagrees
Amgen, Tavneos’ manufacturer, attributes the enrollment shortfall to challenges in recruiting patients already receiving the drug. In a June letter, Amgen claimed Tavneos’ benefits outweigh its risks. The FDA strongly disagrees, stating in an April letter that it “can no longer conclude that there is, or has ever been, a valid demonstration of substantial evidence of effectiveness” for the drug’s approved use. The agency cited 76 cases of drug-induced liver injury and warned that without proven efficacy, Tavneos’ risks outweigh its benefits. FDA officials are seeking to remove the drug from the market, a move Amgen is contesting.
The FDA’s enforcement actions vary. A spokesperson for HHS noted that delays may stem from legitimate technical issues, though the agency has issued “failure to respond” letters in some cases, such as with Pfizer’s Paxlovid misuse study. In other instances, like the CustomFlex Artificial Iris device, zero patients have enrolled for a pediatric study approved in 2019, prompting efforts to find alternative compliance methods.
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Other Delayed Studies Highlight Systemic Issues
A KFF Health News review of FDA databases revealed nearly 600 delayed postmarket studies involving almost 350 products. These delays span medications like Paxlovid, where a study on pregnancy safety missed its 2024 deadline. Pfizer cited “submitting results as soon as practicable” while the study remains incomplete. The CustomFlex Artificial Iris device, approved for pediatric use in 2019, has enrolled zero patients for its five-year follow-up, with stakeholders exploring alternative compliance methods due to the rarity of aniridia.
The database also shows the Scandinavian Total Ankle Replacement system enrolled just 142 of its targeted 500 patients, with nearly half experiencing adverse events. Similarly, Oxaydo, an abuse-deterrent opioid approved in 2011, was discontinued in 2023 after its postmarket study on misuse remained unresolved. The FDA issued a failure-to-respond letter in 2022, and Acura Pharmaceuticals cited expiring patents as the reason for halting sales.
FDA Enforcement and the Exondys 51 Case
The FDA’s enforcement tools vary widely. While the agency can issue failure-to-respond letters or seek court action, its ability to remove products post-approval remains limited. Dr. Ellis Unger, a former FDA official, warned in 2016 that the agency had never withdrawn a drug for failed postmarket verification. This caution echoes with Exondys 51, a treatment for Duchenne muscular dystrophy approved in 2016 despite internal FDA objections. Scientists had deemed the drug unproven, with Unger cautioning it offered “false hope” to patients.
Sarepta Therapeutics, the manufacturer, admitted the drug’s clinical benefit was unverified at approval. The postmarket study, originally due in May 2021, remains delayed, with the FDA database citing extension requests as the reason. In 2022, Exondys 51 ranked as the second-most expensive drug in the U.S., costing up to $1.5 million annually. Despite enrollment challenges, the study fully enrolled by 2023, with initial results expected late 2026. Sarepta generated $538 million from the drug in 2022 alone, highlighting the financial stakes of delayed oversight.
The FDA continues pressuring manufacturers to complete studies. For Tavneos, the agency insists the drug lacks proven effectiveness, citing 76 liver injury cases. Amgen disputes this, maintaining the benefits outweigh risks. The FDA’s April letter stated it “can no longer conclude” the drug demonstrated substantial evidence of effectiveness. With global sales of $459 million last year, Amgen’s effort to keep the drug highlights the tension between regulatory oversight and industry profitability.
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