Mesalazine, also known as 5-aminosalicylic acid or 5-ASA, has been a cornerstone of inflammatory bowel disease management for decades. It is prescribed to both quell active flares of ulcerative colitis and Crohn’s disease and to keep those diseases in remission over the long term. Because it is generally well tolerated, patients often take it for years with little thought. Yet the new report underscores that even this familiar molecule can provoke serious lung injury. Drug-induced eosinophilic pneumonia occurs when an offending medication triggers an influx of eosinophils into the alveolar air spaces and the interstitial tissue of the lungs, impairing oxygen exchange and producing cough, breathlessness, and infiltrates visible on chest imaging. Diagnosis typically rests on a compatible drug history, elevated eosinophils in peripheral blood or bronchoalveolar lavage fluid, characteristic imaging, and exclusion of infection and other causes.

What makes this case exceptional is timing. In the vast majority of reported instances, mesalazine-related lung injury emerges two to six months after treatment begins, a lag that can easily lull clinicians into overlooking the drug as a culprit. The literature contains only a handful of cases in which symptoms appeared within ten days of the first dose, and the present patient, whose cough began on day seven, now ranks among the earliest ever recorded. By the time she was referred to hospital on day 39, chest radiography showed worsening consolidation in the right upper lung field along with a pleural effusion. Computed tomography demonstrated diffuse consolidation concentrated in the right upper lobe. Her vital signs remained stable, with an oxygen saturation of 96 percent on room air, but her white cell differential told a striking story: eosinophils made up 37.8 percent of a total count of 9,700 cells per microliter, and her C-reactive protein was mildly elevated at 2.67 mg/dL.

To pin down the diagnosis, clinicians turned to bronchoscopy. Bronchoalveolar lavage returned fluid with a total cell count of 5.88 × 10⁶ per milliliter, of which an extraordinary 78.6 percent were eosinophils, a signature finding in eosinophilic lung disease. Transbronchial biopsy and transbronchial lung cryobiopsy, a technique that freezes a larger sample of lung tissue for histological analysis, revealed dense eosinophilic infiltration in both the alveolar spaces and the interstitium. Bacterial and mycobacterial cultures and cytology were all negative, ruling out infection and malignancy. With the drug withdrawn, the clinical course became its own confirmation: chest radiography began improving just two days after mesalazine was stopped, the patient’s cough resolved within a month, and pulmonary infiltrates and peripheral eosinophilia had normalized six weeks later, all without corticosteroid therapy. She was subsequently transitioned to vedolizumab, a gut-selective biologic, for her ulcerative colitis, and the pneumonia did not recur.

Behind this single patient stands a much larger body of evidence assembled by the reporting team. Searching MEDLINE and screening references from prior reviews, the authors identified 57 additional cases of mesalazine-induced lung injury, bringing the total analyzed to 58. The demographic picture is instructive: patients ranged from 14 to 84 years of age, with a median of 35 and a peak in the third decade of life, and women outnumbered men 34 to 24. Ulcerative colitis accounted for 84 percent of underlying diagnoses, with Crohn’s disease making up most of the remainder. Eosinophilic pneumonia was the most common pattern of injury, representing 43 percent of cases, followed by interstitial pneumonia at 27 percent and organizing pneumonia at 10 percent. Most patients, 67 percent, showed bilateral infiltrates, though unilateral disease, as in this case, does occur and can mislead clinicians toward an infectious diagnosis.

The timing data from that literature review are perhaps the most clinically valuable takeaway. While lung injury typically develops two to six weeks after initiation, roughly 10 percent of cases occur within the first two weeks, and only two cases on record, including this one, manifested within ten days. In a focused analysis of 18 well-documented eosinophilic pneumonia cases, symptom onset ranged from seven days to fourteen months, with 70 percent occurring within two months. The median cumulative dose at onset was 81 grams of mesalazine; the present patient had consumed only about 14 grams when her symptoms began. That discrepancy carries real mechanistic weight, because a reaction that ignores both the duration of exposure and the total dose ingested is the hallmark of hypersensitivity rather than toxicity.

That mechanistic distinction matters for how clinicians think about drug safety. Drug-induced lung injury generally arises through two principal pathways: direct cytotoxic damage to alveolar epithelial or endothelial cells, which tends to be dose-related, and immune-mediated inflammation, which can erupt unpredictably at any exposure level. The evidence points firmly toward the immune pathway for mesalazine. Researchers propose that the drug may skew immune signaling toward a Th2-dominant response, in which cytokines such as interleukin-5 drive the production, recruitment, and activation of eosinophils, ultimately seeding them in lung tissue. Pharmacokinetic data reinforce the plausibility of a hypersensitivity mechanism: approximately 20 to 30 percent of orally administered mesalazine and about 10 percent of rectal formulations are absorbed systemically, and eosinophilic pneumonia has been reported even after rectal administration, indicating that small systemic exposures can suffice to ignite the reaction.

The case also settles a long-standing question about the older drug sulfasalazine, a prodrug that is metabolized in the gut into mesalazine and sulfapyridine. When eosinophilic pneumonia occurred in patients taking sulfasalazine, toxicity was often attributed to the sulfapyridine moiety. But the accumulating reports of identical lung injury with mesalazine alone suggest that the 5-aminosalicylic acid molecule itself is capable of triggering the immune response. For the millions of patients with inflammatory bowel disease who take 5-ASA compounds worldwide, this reframing means that no formulation of the drug class can be considered free of pulmonary hypersensitivity risk, however rare that risk may be in absolute terms.

One intriguing wrinkle in the present case is the patient’s respiratory history. She had suspected bronchial asthma and used inhaled corticosteroids as needed, and her fractional exhaled nitric oxide, a noninvasive marker of eosinophilic airway inflammation, was elevated at 51 parts per billion. The authors caution that this reading may have reflected pre-existing asthmatic airway inflammation rather than pneumonia alone, and they note that only one previously reported mesalazine lung-injury patient had a history of bronchial asthma. Whether underlying allergic airway disease can accelerate the onset of drug-induced eosinophilic pneumonia remains uncertain, but the possibility offers a concrete hypothesis for future research and a reason for heightened vigilance in asthmatic patients starting the drug.

The practical message for clinicians and patients alike is one of awareness rather than alarm. Corticosteroids, the usual mainstay of eosinophilic pneumonia treatment, are not always necessary; simple drug withdrawal can be sufficient when respiratory status is stable, as this case demonstrates. Yet some patients have received prednisolone out of concern that stopping mesalazine might precipitate a relapse of their inflammatory bowel disease, illustrating the delicate balancing act physicians face. The authors’ conclusion is straightforward: eosinophilic pneumonia can develop early in the treatment course, and drug-induced lung injury should be considered whenever respiratory symptoms or pulmonary infiltrates appear during mesalazine therapy. For a drug taken daily by so many, recognizing that the lungs can protest within a single week of the first tablet may make the difference between a swift, uncomplicated recovery and a prolonged diagnostic odyssey.

Subject of Research: Rapid-onset mesalazine-induced eosinophilic pneumonia occurring seven days after drug initiation

Article Title: Rapid Onset of Mesalazine‐Induced Eosinophilic Pneumonia Manifesting 7 Days After Initiation: A Case Report

Article References: Inazaki, T., Takeda, K., Iwasaki, M., Tajima, H., Shionoya, Y., Hirama, R., Sato, S., Naito, A., Kawasaki, T., Ikari, J., Kageyama, S., Ikeda, J.-I., & Suzuki, T. (2026). Rapid Onset of Mesalazine‐Induced Eosinophilic Pneumonia Manifesting 7 Days After Initiation: A Case Report. Respirology Case Reports, 14(9), Article e70758. https://doi.org/10.1002/rcr2.70758

Image Credits: AI Generated

DOI: 10.1002/rcr2.70758

Keywords: mesalazine, eosinophilic pneumonia, ulcerative colitis, drug-induced lung injury, inflammatory bowel disease, bronchoalveolar lavage, hypersensitivity pneumonitis, 5-aminosalicylic acid, adverse drug reaction, respirology, eosinophils, case report