This study aimed to investigate the effectiveness of pharmacist intervention in reducing and preventing prescribing errors of investigational drugs for cancer patients. A retrospective study was conducted during two periods: a baseline period from December 2015 to June 2016 and an intervention period from July 2016 to February 2017. The investigational drug service (IDS) pharmacists performed active interventions during the intervention period. Among 12,387 investigational drug orders, 395 (6.1%) prescribing errors were detected in 6477 orders at the baseline period, and 278 errors (4.7%) were detected in 5,910 orders at the intervention period. To identify factors that affect prescribing errors, three models were constructed for the multivariate analysis. Among factors affecting prescribing errors, sponsor initiated trial (SIT) was the strongest factor (AOR: 4.16, 95% CI: 3.31-5.23). Pharmacist intervention reduced prescribing errors by at least 25% in all constructed models after adjusting for confounding variables. Prescribing errors were 1.3 times higher when dealing with intravenous medications than when dealing with oral medications. There were 60% fewer prescribing errors in the blinded study than in the open study. SIT and multi-center/multi-nation studies had 4.2 and 2.4 times more frequent prescribing errors than in investigator-initiated trials (IIT) and single-center/single-nation studies, respectively. Fewer errors occurred in phase 2 and trials covering both phase 1 and phase 2 (phase 1/2) than in phase 3 trials. The IDS pharmacist intervention in cancer clinical trials was associated with significant reductions in prescribing errors and may lead to increased medication safety.