Since the discovery of HIV, scientists have made major inroads in understanding modes of transmission, infectivity, and pathogenicity. Therapeutic alternatives, especially antiretroviral drugs, have been tested and approved and are providing benefit to many who are HIV-infected.

Opioid analgesics are approved by the FDA for the treatment of moderate or severe pain. However, individual patients differ greatly in clinical response to different opioid analgesics, and patient populations show widely variable response to the same opioid and dose.