Figure 4. Dose-response curve. Dose-response curve for intrathecal fentanyl for intrathecal labor analgesia. Effective analgesia ("pain relief") was defined as a VAS pain score < 20 at both 10 and 40 min after injection. The line is defined by the: Equation 3. The ED50of intrathecal fentanyl for labor analgesia is 14 micro gram (95% confidence interval, 13–15 micro gram).