Background Some studies in the literature describe drug-related problems in patients with cancer, although few studies focused on patients receiving targeted chemotherapy and/or immunotherapy. To identify the incidence of drug-related problems in patients receiving targeted chemotherapy and/or immunotherapy, and demonstrate the impact of a clinical pharmacist in an outpatient oncology care setting. Methods Prospective study was conducted in a hospital outpatient oncology clinic between October 2015 and March 2016. Patients greater than 18 years old receiving cetuximab, nivolumab, ipilimumab, or pembrolizumab were included in the study and monitored over a three-month period by a clinical pharmacist. Drug-related problems were analyzed using the Pharmaceutical Care Network Europe classification system. The main outcome measures were the frequency and causes of drug-related problems and the degree of resolution achieved through the involvement of a clinical pharmacist. Results A total of 54 patients (mean age: 57 +/- 12 years) were included. There were 105 drug-related problems and 159 associated causes. Among the planned interventions (n = 149), 92 interventions were at the patient-level with 88 (96%) being accepted by the doctors. This resulted in 68 (65%) drug-related problems being completely resolved and 9 (8.6%) being partially resolved. The most common drug-related problem identified was "adverse drug event" (n = 38, 36%). Of the 105 drug-related problems, 63 (60%) related to targeted chemotherapy and/or immunotherapy with 34 (54%) classified as an "adverse drug event." Conclusion Adverse drug events were the most common drug-related problems in patients with cancer. The involvement of a clinical pharmacist improved the identification of drug-related problems and helped optimize treatment outcomes in patients receiving targeted chemotherapy/immunotherapy.