Adjuvant analgesics are drugs that have weak or nonexistent analgesic action when
administered alone, but can enhance analgesic action when co-administered with known
analgesic agents. Ketamine, an anesthetic drug, is an adjuvant analgesic drug. Its use in doses
lower than therapeutic doses might be important in the management of certain types of pain as
neuropathic pain.
The present study was performed to investigate the effects of subanesthetic doses of ketamine
in four animal models of induced pain and to compare its effects with diclofenac sodium. It is
also intended to test the effectiveness of combining both drugs together in these animal models
of pain.
All experiments were performed on albino mice. Mice were evaluated for their responsiveness
to noxious stimuli using four tests: tail-flick test, hot-plate test, formalin test and acetic acidinduced
writhing test. These effects were measured before and one hour after intraperitoneal
drug administration. In some experiments, they were followed for 6 and 24 hours.
We found that Ketamine, in subanesthetic doses, resulted in a significant analgesic effect in all
the four models of pain. It increased pain latencies in tail-flick test by 78% compared to preadministration
time, and by 95% in hot plate test. It also decreased the number of lickings and
bitings in formalin test by 41.9% and the number of abdominal writhing by 73.5%. These
analgesic effects represented around 60% of diclofenac effect in heat-induced pain models, but
it is similar to diclofenac in the other two models. The enhancement of diclofenac analgesic
effect by ketamine ranged from 13.6% to 46% in the four tests.
It is concluded that Ketamine in subanesthetic doses has a significant analgesic effect
comparable to diclofenac. It can enhance diclofenac effect by a margin not exceeding 50% of
diclofenac effect. Much smaller doses of ketamine are required to be tested in the future.