It started innocently enough. How could the cure to all pain not be seen as the highest achievement mankind had attained? Diabetic Neuropathy, chronic pain due to nerve injuries, childbirth, sudden injuries, Fibromyalgia, the list of issues the medicine cured was endless. Mental pain was even alleviated by this "miracle drug", Nondolere. Who could - who _would_ have seen the consequences of such a life send? Every household was given a month's worth of doses.
The path to Hell is paved with good intentions, so they say. Among the parades and galas and ceremonies and all the other praises celebrating Nondolere, there were whispers of problems. Of course the media never reported on problems. Everyone seemed to have an uncle or grandmother or little cousin who had died from seemingly routine illnesses. There were talks of some people dying within hours to few days after taking it. All these were attributed to urban legends. Nondolere was available to everyone, regardless of insurance or lack thereof. The medicine stopped pain; not death. It helped the whole of humanity.
Some (mostly conspiracy theorists and those who mistrusted the government) threw away their prescriptions and declined to continue using the Nondolere. Many of those were unheard-ofs who had no influence, therefore were inconsequential. Any public figures who naysayed the drug were quickly turned to pariahs. As far as the general public was concerned everything was fine; all systems go.
Over time the deaths became more publicized. The media nor the government could prevent what was happening. A car accident victim walked away from certain death, only to die the next day at the supermarket. Women died in childbirth; as did their unborn children. People were murdered randomly. Some people dropped dead in the street for no apparent reason. Society barely felt the fractures as civilization began to crumble. Obsession with not feeling pain overtook any interests the average person had with... anything.
By the time the damage was done it was far too late. Those in the medical field -mostly researchers- identified the addictive properties of Nondolere, along with several other severe risks including lack of emotional feeling, rage, suicidal ideation, and homicidal ideation. As it turned out, the drug hit not only pain receptors but pleasure centers and every dose of the drug reinforced the "feel goods" the users' brains felt. A small portion of those who took the drug had an adverse reaction and became unceasingly violent. The result was billions of people who heard claims of issues with the drug within the first few days, but who didn't care.
After 4 days, those who remained because of the initial fear of the drug were feeling the depression of loss. They broke and took a dose. At day six, murders and suicides skyrocketed. By day 7, 90% of the human population was dead. Those still remaining on the dawn of that fateful 7th day were too far gone to last long. The last of the Earth's inhabitants went out with a whimper; sucking up the remainder of the drug that cured humanity.