The transdermal fentanyl patch (TDF) can be used when switching from other opioids; therefore, little is known about the efficacy and safety of TDF patches applied for opioid initiation. However, TDF patches have been applied for opioid initiation in gastrointestinal cancer patients with gastrointestinal obstruction. In this study, we retrospectively investigated 12 gastrointestinal cancer patients to evaluate the efficacy and frequency of adverse effects of TDF patches compared to oral oxycodone (OXY) for opioid initiation. The frequency of adverse effects such as nausea, somnolence, and constipation in the TDF patch group was 25%, 41.7%, and 8.3%, respectively. No severe adverse effects were observed, and there was no significant difference between the TDF patch and OXY groups. Moreover, according to the numerical pain rating scale(ranging from 0 [no pain] to 10 [worst possible pain]), the pain intensity in the TDF patch group decreased from 5.42 on the first day to 3.33 after 3 days (p=0.0377), and 2.67 after 7 days (p=0.0089), with no significant difference between groups. Our study results suggest that TDF patches applied for opioid initiation may be useful for gastrointestinal cancer patients with gastrointestinal obstruction.