Opioids alter paw placement during walking, confounding assessment of analgesic efficacy in a postsurgical pain model in mice.

Unfortunately, the opioid-induced "tiptoe" gait phenotype prevented gait analysis from being a viable metric for demonstrating opioid analgesia in injured mice. However, this work reveals an important, previously uncharacterized effect of treatment with analgesic doses of opioids on paw placement. Our characterization of how opioids affect gait has important implications for the use of mice to study opioid pharmacology and suggests that scientists should use caution when using hind paw-directed nociceptive assays to test opioid analgesia in mice.