Non-racemic mixtures of D- and L-methadone containing a ratio ranging from about 2.5:1 to about 3.5:1 by weight of D-methadone to L-methadone, such as for example about 2.9:1 to about 3.1:1 by weight of D-methadone to L-methadone, or about 3:1 by weight of D-methadone to L-methadone, have been found to exhibit surprising and unexpected beneficial results in the treatment of neuropathic pain. Additionally, non-racemic mixtures of D- and L-methadone containing a ratio ranging from about 2.5:1 to about 3.5:1 by weight of D-methadone to L-methadone, about 2.9:1 to about 3.1:1 by weight of D-methadone to L-methadone, or for example about 3:1 by weight of D-methadone to L-methadone, in combination with other non-methadone opioids have been found to exhibit surprising and unexpected beneficial results in the treatment of mixed pain.