You asked about opioid ABUSE before/during the Civil War, and, well, while not nearly as common as the years after the war as a surge of veterans attempted to resume normal life, opioid addiction was a significant public health problem even before the war. The potential for opioids to treat and relieve pain was recognized in the early 19th century. Without the understanding we have today regarding their dangers, doctors often prescribed copious amounts of opioids in the form of morphine, laudanum (opioid powders mixed with alcohol), and yes, opium, to their patients as a sort of cure-all. The trouble with this is, even when starting at a reasonable dose, long-term opiate users develop a tolerance, needing more and more to achieve the same effects or suffering withdrawals. Thus, as early as the 1830's, and rising steadily up until the start of the war, opioid abuse (or 'Opium Eating', as the press described it at the time) was certainly a significant concern among people from all walks of life. Before the war, the 'Opium Habit' was recognized as a disease, as well as the dangers of accidental overdoses, but the ubiquitousness of these drugs meant that doctors were reluctant to stop prescribing them. The fact that wealthy white women were often most prone to addiction- prescribed opioids for 'female troubles' as well as what they called 'nerves'- meant that, while still recognized as a negative thing, addiction held much less stigma than how we would consider IV users of opioids today.

During the Civil War, opioid use was absolutely commonplace as a treatment for wounds sustained in battle. The invention of new weaponry and the sheer scale of the injuries occurred made opioids a necessity for doctors of the day. These medicines were used for pain relief, absolutely, but doctors also used them to suppress symptoms of illness such as coughing or retching, opioids being a depressant and all. Battlefield hospitals were essentially breeding grounds for outbreaks of contagious disease, so even if you didn't get injured in the war, it's still possible you could be given some morphine and get hooked.

After the war, what was very likely tens of thousands of soldiers returned home addicted to painkillers. The shift in demographics from wealthy white women to young or middle-aged men as the face of addiction fostered increased stigma towards addicts. It was especially shameful for men because addiction was often described as a 'slavery' or 'bondage'. I can't speak to the racial tones here, but this was a significant blow to their masculinity, with independence and self-sufficiency synonymous with manhood and moral standing. Most people would have likely held the opinion that addiction was a fault in an individuals' character, allowing themselves to be controlled by an outside force. The wave of addicted soldiers returning home could not be ignored, and in the decades following the war, physicians began to prescribe opioids less and less, partly because many soldiers blamed them for kickstarting their addiction. With fewer opioids available to the public, smoking opium (rather than IV or oral use) became increasingly popular beginning in the 1870s, and opium dens were frequently patronized by Chinese immigrants and young, white, lower-class males. This marked a profound shift in how Americans at the time viewed addiction- immoral, lower-class, and shameful.