Palliative care physicians are accustomed to using transdermal fentanyl patch for cancer pain control but not so familiar with its intravenous administration. Historically, fentanyl has been used to treat breakthrough pain because it is a very potent analgesic with a rapid onset and short duration of action. Although several formulations of fentanyl have been recently developed for breakthrough cancer pain, these are unavailable or too expensive in some countries. Also, all opioids can induce tolerance potentially and different opioids cause significantly different degrees of tolerance. Therefore, sequential opioid trials may be a reasonable approach in patients with poor analgesic effect after dose escalation. Here, we present 2 morphine-tolerant patients with cancer in whom the intravenous fentanyl was effectively used for their refractory breakthrough pain.