Opioids attach to opioid receptors in the brain and other parts of the body. When bound to the receptors, opioids block the pain signals in the body which is what makes them so effective as pain relievers. However, much like alcohol, the tolerance for these drugs and their relieving effects becomes greater and greater, thus setting up a cycle of potential abuse for those who wish to chase the initial potency of the drugs.
What is often not discussed is how the cessation of pain due to opioid usage and abuse can actually lower the pain threshold of patients who are repeatedly using opioids. So, not only is there a cycle of abuse inherent in drugs that users develop tolerance for (until fatal dosing levels are reached), but the initial need which may have been founded on chronic pain has now become significantly worse due to a lowered ability to tolerate pain comfortably and a sensitivity to pain is founded.
Like with alcohol addiction, the progression is such that a person must drink to feel normal. In the case of opioid addiction, this is also true. As pain and discomfort become dominant when not under the influence. However, “normal” is the wrong choice of wording when referring a drug induced homeostatic response. The barrage of opioids in a person’s body can result in both short and long term effects.