Introduction The treatment of non-small cell lung cancer (NSCLC) has profoundly changed on account of the arrival of new therapies, like immunotherapy. Within this group of drugs, those aimed at the programmed cell death-1 or programmed cell death ligand-1(PD1/PDL-1) are very relevant, for example, Pembrolizumab. Although its adverse reactions are generally mild and well tolerated, it has been associated with certain immune-related adverse events (IrAEs) than can be serious and affect any organ. Case report A 62-year-old woman diagnosed with stage IV NSCLC with a single bone metastasis and PD-L1 expression of 60% started treatment with cisplatin-pemetrexed-pembrolizumab, and maintenance with pembrolizumab. Management and outcome The patient attended the ER with pericardial effusion that was assumed to be a Pembrolizumab IrAE and was managed with corticosteroids. The patient fully recovered but immunotherapy was not reintroduced due to the severity of the AE. Discussion The cardiovascular system is among the least affected organs by immunotoxicity, with an incidence between 0.09–0.6%. However, some authors suspect the incidence is underestimated. Median time to onset is highly variable, ranging from 6 weeks since the first dose to 2 years after discontinuation of the treatment. There are not guidelines on the most effective management of the IrAEs, but according to the pharmaceutical reference, corticosteroids should be initiated followed by a progressive reduction. If no response is obtained, another immunosuppressive agent should be added. The determination to restart immunotherapy depends on the severity of the adverse reaction, the availability of other alternative treatments, and the cancer response.