A recent study performed in rats indicates that a brief high dose of opioid drugs may erase memory traces of pain in the spinal cord (Drdla-Schutting R et al. Science. 2012;335:235-238).
Opioids exert their pain-relieving effects by binding to μ-opiate receptors on nerve cells. Opioids are generally given continuously in moderate doses to achieve permanent binding to these receptors because the drugs are thought to alleviate pain only while bound.
Hampton T. Quick Hit of Opioids. JAMA. 2012;307(9):904. doi:10.1001/jama.2012.223