Because acute pathology and long-term behavioral change occurred in the same dose range as antihyperalgesic effects, the therapeutic ratio of intrathecal ketamine is less than one in the neonatal rat. This measure facilitates comparison of the relative safety of spinally administered analgesic agents.
Walker, S. M., Westin, B. D., Deumens, R., Grafe, M., & Yaksh, T. L. (2010). Effects of intrathecal ketamine in the neonatal rat: evaluation of apoptosis and long-term functional outcome. Anesthesiology, 113(1), 147-159. https://doi.org/10.1097/ALN.0b013e3181dcd71c (Original work published 2010)