There are surprisingly quite a few. In my case I have issues with discs in my spine and neck.

Sometimes it can be an autoimmune condition like multiple sclerosis or rheumatoid arthritis. Sometimes it can be downright mysterious such as many cases of fibromyalgia.

Mild sciatica is kind of funny but if you get a badly pinched nerve it is excruciating.

Opiates turned off the pain for 2 hours out of three, and for a third hour I suffered. I did not get a full night's sleep for 6 weeks.

Severe pain makes your life a living hell and is responsible for a lot of suicides.

I don't take pain killers now because my back got better, but I have other needs for controversial medications. It is a life of constant anxiety as the government keeps changing the rules. You never know from one appointment to the next whether the system will throw you under the bus again.