Background Drug-related problems (DRPs) prevent patients from fully benefiting from drug treatment. Unrelieved pain in patients with cancer is still widespread. Pharmacists can play a role in closely monitoring cancer patients, pain control maintenance, and patient consultation. Objective To evaluate the clinical effects and changes in drug costs of pharmacists' interventions on patients with DRPs related to cancer pain. Setting An academic teaching hospital in Shanghai, China. Methods Patients with cancer pain admitted to Shanghai Tongren Hospital from October 2018 to February 2019 were randomized into the intervention and control groups. The Pharmaceutical Care Network Europe classification V8.02 was used to categorize DRPs treated with analgesics. Patients' pain relief, the occurrence of adverse drug reactions, and drug cost-saving through the resolution of DRPs were evaluated. Main outcome measure Problems and causes of drug-related problems, interventions proposed, and outcome of pharmacy recommendations. Results A total of 172 patients were enrolled and randomized into the intervention group (n = 86) and the control group (n = 86). The pharmacist detected 66 DRPs in 48 patients (55.8%) of the intervention group, an average of 0.8 DRPs per patient. A total of 149 interventions were proposed by the pharmacist. Compared to the control group, the drug intervention produced more pain relief on the third day of analgesic treatment. In the intervention group, a total of 33 DRP interventions resulted in cost changes, saving a drug cost of $489.90, averaging $11.94 per intervention. Conclusion Our study suggests that pharmacy service in patients with cancer pain can resolve drug-related problems and reduce drug costs.