The reason for differences in molar concentration are two-fold. Levobupivacaine is presented commercially as a base and not as a hydrochloride and therefore has 12.7% more molecules than the equivalent concentration of bupivacaine. In addition, ropivacaine has a smaller molecular weight than either drug and thus has 4.5% more molecules than bupivacaine and 8.4% less molecules than levobupivacaine. Although molar differences between drugs are relatively slight, a small reduction in molar concentration may provide similar pain relief, but result in a large reduction in side effects due to the shape and position of the efficacy and side effect dose response curves.