To be clear, the evidence to suggest opioids have a meaningful impact on immunity is quite scant.  There are no prospective  clinical trials to support that notion.  Patients on higher dose opioids have poor immunity more often than not, because their disease is more advanced (and marrow is suppressed).  There also is not good evidence, thus far, that marijuana can substitute for opioid analgesia, especially in cancer patients.  THC compounds are definitely helpful to some patients, especially with sleep disturbance, nausea, poor appetite, or nerve pain (neuropathic pain).  Opioid requirements may be reduced somewhat for regular uses.  Likewise, the move to substitute opioids with marijuana in chronic non-cancer pain is an approach many primary care docs have adopted.  However, what are the long term toxicities/side effects from this approach?  Will it provide better or similar pain control? We don't know. Addiction risks?