The most commonly used vascular access is the peripheral intravenous catheter (PIVC). However, it can trigger complications and the occurrence of adverse events, such as phlebitis. This study evaluated the variables that are associated with the occurrence of phlebitis in medical and surgical inpatient units. This is an observational, retrospective, case-control study in medical and surgical hospitalization units of a private general hospital in the city of São Paulo. Participants were an average age of 66.3 years, and 71% were hospitalized in medical units. The risk variables associated with phlebitis were medical hospitalization (odds ratio [OR] = 4.36; P = .002), presence of comorbidity (OR = 10.73; P < .001), and having 5 or more PIVCs (OR = 53.79; P = .001). Regarding intravenous therapy, the use of contrast was a risk variable (OR = 2.23; P = .072). On the other hand, patient education regarding PIVCs was a protective measure against the development of phlebitis. The nursing team plays an essential role in the care of patients with PIVCs, inpatient guidance, planning, and device choice, taking into account the risk factors for phlebitis to maintain the preservation of vascular health and reduce adverse events.