Objective To establish a method for evaluating the complexity of clinical trials (CTs) from the perspective of a pharmacy service (PS) and to analyse the complexity of CTs carried out in a tertiary level hospital.

Methods An observational, prevalence and retrospective study was carried out in a Spanish tertiary level hospital during the period 2008–2013. A scale of complexity was developed, whose internal consistency was determined by Cronbach’s alpha. The study involved five steps: an analysis of the activities involved, score allocation to the activities, identification of CTs started in the study period, data collection and assessment of the complexity. Three complexity levels were determined: low, medium and high. The variables calculated were mean overall complexity, mean complexity per medical specialty, per pathology, per phase of CT, per initiation year and percentage of CTs by complexity level.

Results Cronbach’s alpha of the scale of complexity was 0.738. The two most influential items were dose preparation and number of professionals involved. 55.0% of CTs were in the medium level of complexity and 12.1% of CTs were in the high level. The mean complexity of CTs studied was 13.3±4.7 (median 12, range 6–32). Statistically significant differences were found in the complexity values between CTs of different medical specialties, pathologies, phase and dose preparation in the PS (p<0.001).

Conclusions The scale designed to evaluate the complexity of CTs had internal consistency. More than half of the CTs are in the medium level if complexity. The largest number of CTs with a high level of complexity were in rheumatology and oncology.