Objective To determine the pharmaceutical interventions in patients eligible for phase I cancer clinical trials, focusing specifically on exclusion criteria related to medication or relevant interactions.

Method Descriptive, observational study conducted at a comprehensive cancer centre. Patients undergoing screening for phase I clinical trials (March 2019–December 2022) were included. The pharmacist reviewed concomitant medication and provided a recommendation.

Results The concomitant medication of 512 patients eligible to participate in 84 phase I clinical trials was analysed. In 230 (44.9%) patients, the clinical trial treatment included oral medication. The median number of concomitant medications was 5 (IQR 3–8) per patient.

A total of 280 pharmaceutical interventions were performed in 140 (27.3%) patients: 240 (85.7%) were due to interactions in 124 (24.2%) patients, and 40 (14.3%) were due to exclusion criteria in 34 (6.6%) patients. Interactions and exclusion criteria were detected in 18 (3.5%) patients. The main groups of drugs involved were 68 (24.3%) antacids and antiulcer drugs, 28 (10.0%) antidepressants and 26 (9.3%) opioids. Acceptance analysis of the recommendation was applicable in 215 cases; in 208 (96.7%), the pharmaceutical intervention was accepted.

Differences were identified for exclusion criteria (7 vs 27) and interactions (37 vs 87) between parenteral and oral clinical trial medication (p<0.001).

Conclusion The pharmacist’s review of concomitant medication during the screening period in phase I clinical trials enables the detection of prohibited medication or relevant interactions, potentially avoiding screening failures and increasing the efficacy and safety of treatments.