The randomized AMBORA trial showed that medication errors are frequent in patients treated with oral antitumor therapeutics and that they can be substantially reduced by an intensified clinical pharmacological/pharmaceutical care program. While randomized controlled trials are essential to generate clinical evidence, their generalizability in real-world is not always given. The AMBORA care program was implemented in clinical routine within the AMBORA Competence and Consultation Center (AMBORA Center) at the Comprehensive Cancer Center Erlangen-EMN, allowing a thorough comparison of medication error frequencies and characteristics. Our primary analysis compared data at therapy initiation of new oral antitumor therapeutics from the AMBORA trial intervention group (n = 98) and the AMBORA Center (n = 142). Medication errors involving the oral antitumor therapeutics were twofold higher in real-world compared to the randomized controlled trial (mean 0.83 ± 0.80 per patient vs. 0.41 ± 0.53, P < 0.001). We observed more complex oral antitumor therapeutic regimens, a higher median number of medications, and a higher ECOG status in clinical routine vs. the randomized trial. A high percentage of medication errors was completely solved in both groups (85.7% vs. 88.3%, ns). Medication error characteristics within the complete medication (oralantitumor therapeutics and concomitant medication) were similar in both groups (e.g., patient-related causes, drug-drug/drug-food interactions). Taken together, medication errors were even more frequent in clinical routine than in the randomized controlled trial and a high rate was solved in clinical routine by a clinical pharmacological/pharmaceutical care program.