These drugs were not originally designed or licensed for pain relief and certainly not for injection to the Intrathecal Space of the spinal cord. In fact the manufacturers knowing that they are used for this purpose have included large warnings on the Package Inserts against such use. Over the years, and, most commonly for birthing mothers they have been used in this way routinely.
Many Pain Specialists will recommend these to people with Low Back Pain (LBP) long before they have any idea of what is causing it. In fact it is regularly used before the patient has had imaging studies carried out. More expensive again.
The net result, in my own experience, is that they pain they were supposed to relieve either instantly, or over time, doubles or trebles because the patient develops ARC as a result, if they don't already have it. They can also cause other undesirable effects.
One of the worst is meningitis, usually occurring because the hole in the spinal cord has not closed properly and the patient contracts the infection from their own skin surface, no matter how much iodine is used. The first sign of this is a massive headache due as much to the loss of Cerebral Spinal Fluid as the infection and the first treatment for that headache is to inject 5ccs of the patient's own blood into the hole. You get 10 bonuspoints for telling me which of the two is the worst potentiating agent of ARC. Got it? Human blood! Give the boys in white some "brownie" points.