Addiction to specific drugs such as alcohol, psychostimulants, and opioids shares some common direct or downstream effects on the brain's stress-responsive systems, including arginine vasopressin and its V1b receptors, dynorphin and the <i>κ</i>-opioid receptors, pro-opiomelanocortin/<i>β</i>-endorphin and the <i>μ</i>-opioid receptors, and the endocannabinoids.