Different medications can be used as analgesics in labor epidurals. Bupivacaine is one of the most commonly used drugs for that purpose. The efficacy of a certain medication injected epidurally depends on the dose that is given. A certain dose can be administered in different concentrations, which will consequently mean different volumes. Studies have been done to determine volumes for different concentrations of bupivacaine. However, those studies fall short in the sense that this information cannot be readily applicable in clinical practice, as we don't use bupivacaine plain. The combination of bupivacaine and fentanyl or sufentanil is common practice, as it is well established that the opioids will reduce the concentration of local anesthetic required to produce optimal effect with the least side effects.