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Background and Aim: Despite identification of the antinociceptive effects of rosemary and peppermint essential oils, use of these essential oils has been limited due to its low solubility in water and low bioavailability. Use of nanoparticles is one of the ways to overcome these limitations. The aim of this study was to evaluate the antinociceptive effect of nano-emulsions containing rosemary and peppermint essential oils in an animal model of osteoarthritis (OA).
Materials and Methods: In this experimental study nano-emulsions containing rosemary and peppermint essential oils were prepared by spontaneous emulsification. In order to investigate the analgesic effect, 30 male rats were randomly assigned to 5 groups: sham (receiving saline injection into the knee), osteoarthritis(receiving intra-articular injection of 2 mg monosodiumiodoacetate), OA (receiving gels containing nano-emulsion, or rosemary and peppermint essential oil or diclofenac sodium). Treatments were administered topically at a dose of 1 ml daily. Mechanical and thermal allodynia and thermal hyperalgesia tests were performed on the 1^st, 4^th , 7^th  and 14^th  days after injection. Data were analyzed by repeated measures ANOVA.
Results: OA group showed a significant increased behavioral response to the mechanical and thermal stimuli on all days of the experiment compared to the control group (P<0.001). Use of  nano-emulsion or diclofenac gel led to significant increase in the response threshold to von-Frey fillamets (P<0.001), decreased response to acetone (P<0.05), and increased paw withdrawal latency (P<0.05). Histopathology of knee tissues confirmed the results of behavioral tests.
Conclusion: Nano-emulsion containing essential oils of rosemary and peppermint reduced pain of osteoarthritis in rats. The nano-emulsification pro