A real-world disproportionality analysis of adverse events associated with lenvatinib in thyroid cancer patients based on the FAERS database

Thyroid cancer is often a slow-moving disease, but for some patients, it becomes resistant to the standard radioactive iodine treatments that usually control it. When this happens, doctors turn to a powerful medication called lenvatinib. This drug works by blocking the signals that tumors use to grow new blood vessels, effectively starving the cancer. While clinical trials have shown that lenvatinib can extend the lives of these patients, those trials are like a carefully controlled laboratory environment: they involve strict rules about who can join and often end before the very long-term effects of taking the drug for years become clear. In the real world, patients take these medicines for much longer periods, and their bodies may react in ways that were not seen in the initial studies. To understand the full picture of safety, researchers need to look at the vast, unfiltered collection of reports from doctors and patients who have used the drug over time.

A team of researchers turned to a massive digital archive known as the FDA Adverse Event Reporting System to investigate this. This database contains millions of spontaneous reports from around the world, documenting every time a patient experiences a health problem while taking a medication. The researchers focused specifically on reports where lenvatinib was the primary suspect and the patient had thyroid cancer. They sifted through data spanning from early 2015 to late 2025, narrowing the list down to nearly 2,500 detailed reports. Using a set of statistical tools designed to spot unusual patterns, they compared how often specific side effects appeared in these reports against how often they appeared in reports for other drugs. This method allowed them to distinguish between common, expected reactions and signals that might point to new, previously unrecognized risks.

The analysis confirmed several well-known side effects, such as high blood pressure and kidney issues, which are already listed on the drug's official label. However, the study also uncovered seven potential new safety signals that had not been formally linked to the drug before. These included kidney infections, fluid buildup in the abdomen known as ascites, persistent hiccups, pain in the bile ducts, bleeding in the windpipe, seizures, and an unusual increase in red blood cells. While the number of reports for these specific issues was small, the statistical patterns were strong enough to suggest they are not just random coincidences. For instance, the signal for kidney infection was particularly strong, appearing far more often in these reports than would be expected by chance. Similarly, the finding of increased red blood cells, a condition called polycythemia, aligns with emerging evidence from other studies, suggesting it is a real effect of the drug that may have been overlooked because it is not always severe enough to stop treatment.

One of the most revealing aspects of the study was the timing of when these side effects occurred. The researchers found that the risk of adverse events is not constant; it follows a distinct two-stage pattern. Nearly half of all reported problems happened within the very first month of starting the medication. These are the acute reactions, such as high blood pressure or digestive issues, that doctors expect to manage early on. However, the study also identified a second, quieter wave of risk that emerges much later. About 15 percent of the reported events occurred after the patient had been taking the drug for more than a year. These late-onset issues included heart failure, bleeding complications, and headaches. This finding is crucial because it highlights that the safety profile of lenvatinib changes over time. Patients who tolerate the drug well for the first year are not necessarily safe from future complications, as the cumulative effect of the medication can eventually strain the heart and blood vessels.

The researchers emphasize that these findings are based on reports, which means they show a strong association but do not prove that the drug caused every single case. Some of the new signals, like the kidney infections or hiccups, are still considered hypotheses that need further confirmation in future studies. Nevertheless, the data provides a clear map for how doctors might monitor patients more effectively. The study suggests a strategy of intense monitoring during the first month to catch immediate reactions, followed by a steady, long-term watchfulness that continues well beyond the first year. This approach is especially important for thyroid cancer patients, who often stay on treatment for years due to the slow nature of their disease. By recognizing that risks evolve over time and that new side effects can emerge long after treatment begins, medical teams can better protect patients, ensuring they receive the life-extending benefits of the drug while minimizing the hidden dangers that only reveal themselves in the real world.