Antibiotic co-reporting during ICI therapy is associated with a higher reported frequency of irAEs (OR=1.17) and a shorter median time to first reported irAE (31 vs. 42 days) among patients who experienced irAEs, based on analysis of the FAERS database from 2014 to Q4 2024.
Antibiotic use was associated with a significantly higher reported frequency risk of immune-related adverse events in ICI-treated patients.
The strongest antibiotic-irAE associations were observed for specific antibiotic classes including fluoroquinolones, sulfonamides, penicillin, macrolides, cephalosporins, and monobactams.
Antibiotic co-reporting was most strongly associated with higher reported frequency of irAEs in patients receiving PD-L1 inhibitors.
Among patients who reported irAEs, the median time to first reported irAE was shorter in the antibiotic co-reporting group than in the non-co-reporting group.
The pattern of shorter time to first reported irAE with antibiotic co-reporting was most evident in patients receiving PD-1 inhibitors.
The FAERS database analysis has several inherent limitations that preclude causal inference.
This research suggests that cancer patients who take antibiotics while receiving immune checkpoint inhibitor (ICI) therapy — a type of cancer immunotherapy — may be more likely to experience immune-related side effects (called irAEs) compared to those who do not take antibiotics. Analyzing a large U.S. database of adverse drug event reports (over 155,000 ICI-treated patients), the researchers found that antibiotic users had about 17% higher odds of having an immune-related side effect reported, with the association being particularly strong for patients receiving PD-L1 inhibitors (51% higher odds). Certain antibiotic classes — including fluoroquinolones, sulfonamides, penicillin, macrolides, cephalosporins, and monobactams — showed the strongest links to these side effects. Among patients who did experience immune-related side effects, those who also took antibiotics appeared to develop these side effects sooner — a median of 31 days compared to 42 days in those without antibiotic co-use. This timing pattern was most pronounced in patients receiving PD-1 inhibitors. The researchers suggest these findings may be related to antibiotics disrupting the gut microbiome, which is known to influence how the immune system responds to ICI therapy. However, this research suggests these are signals requiring further investigation rather than definitive conclusions. The study relied on a spontaneous reporting database, which cannot confirm the order in which antibiotics and ICIs were given, cannot rule out other explanations (confounding factors), and is not designed for formal analysis of timing. The authors call for carefully designed prospective studies that track medication timing, patient characteristics, and gut microbiome changes to confirm whether antibiotics truly cause earlier or more frequent immune-related side effects in ICI-treated patients.
Yu J, Li Q, Zou S, Rong Y, Zhang Y, Chen C. (2026). Antibiotic use and immune-related adverse events in patients treated with immune checkpoint inhibitors: analysis of the FAERS database.. Frontiers in immunology. https://doi.org/10.3389/fimmu.2026.1733373