Morphine, previously known as a substance of plant origin extracted from poppy flowers, is also produced by the human body as a part of the endogenous opioid system. Endogenous morphine is involved in the regulation of the immune system, helping it return to its baseline state after it has been activated. Clinical and pre-clinical investigations have shown that the administration of morphine leads to immunosuppressive effects, inhibiting the actions of natural killer cells, lymphocyte proliferation and the phagocytic activity of polymorphonuclear cells and monocytes. Additionally, this substance alters the pattern of synthesis and secretion and the paracrine activity of cytokines, favoring the antibody-mediated immune response (Th2- dependent) and inhibiting cell-mediated immune responses (Th1-dependent). These effects are suppressed by naloxone, an antagonist of the miu opioid receptors. Apparently naloxone acts through an isoform called miu- 3, present at the surface of diverse immune cells. Remarkably, the immunotoxic effects observed in HIV negative opium addicts are very similar to those observed in non-addict HIV positive patients who progress to AIDS.