Beta-lactam antibiotics are widely used for the treatment of pneumonia [1], and their use may cause several side effects. The clinical presentation of side effects and their incidence vary depending on the antibiotics, and the severity of side effects is not the same among patients. Antibiotics are usually chosen based on the assumed causative organisms and their estimated susceptibility to antibiotics; the possibility of side effects is often not considered in antibiotic selection.

Diarrhea is one of the most common side effects of antibiotic treatment [2], [3]. The incidence of antibiotic-associated diarrhea (AAD) is reported to be 7–33% [4], [5], [6]. Few studies have reported the risk factors for AAD and non-Clostridium difficile-associated AAD. Age and administration of two or more antibiotics were reported to be risk factors for AAD [7]. Nonetheless, the difference in the incidence of AAD among common beta-lactam antibiotics has not been clarified. Since AAD causes extended hospital stays and increased medical care expenditure and mortality risk [8], [9], [10], avoiding drugs with high risk for AAD is important for appropriate antibiotic selection.

Liver enzyme levels occasionally rise during antibiotic treatment. Drug-induced liver injury (DILI) by all drugs, including antibiotics, is reported to occur in 14–19 per 100,000 people [11], [12]. While most DILIs resolve after removal of the causative agent, some may progress to hepatic failure and result in death. The mortality rate of patients with DILI is 4.7–6% [11], [13]. To minimize the occurrence of DILI, the difference in prevalence among antibiotics and patient-related risk factors for DILI need to be clarified.

Skin rash was observed in 1–8% of patients treated with beta-lactam antibiotics [14]. Severe skin rashes, such as drug reaction with eosinophilia and systemic symptoms syndrome, have been reported as a side effect of antibiotics, and death related to the skin rash was also reported [15]. It has been unclear whether the risk for skin rash is different among antibiotics.

Knowing the differences in incidence and risk factors of side effects according to the characteristics of antibiotics and patients could help in selecting the most appropriate antibiotic treatment. In this study, we elucidate the incidence of side effects associated with antibiotic treatment for pneumonia and the risk factors for each side effect. The hypotheses of this study were that there would be differences in incidence and risk factors among side effects, and the occurrence of side effects would increase mortality. This study was designed to research the incidence of all side effects, reveal risk factors of each side effect, and assess the influence of side effects on prognosis.