Background: One of the most important sources that have been used in treatment of various diseases in human is medicinal plants. Erigeron acer is one of the medicinal plants that used topically to heal toothache and arthritis.

Objective: The aim of this experimental study was to evaluate the analgesic effect of hydroalcoholic extract of Erigeron acer (HEEA) in adult male rats.

Methods: In this study, thirty-six adult male rats divided into six groups: control, under treated with extract (80, 100, 300mg/kg), morphine (1 mg/kg, IP) and naloxone (1 mg/kg, IP)+dose of 300 mg/kg of extract. To evaluate the analgesic effects of extract writhing, tail-flick and formalin tests was used. Acute toxicity of extract also was assessing with method of Lork. 

Results: HEEA at dose of 300 mg/kg clearly has been shown antinociceptive effect in writhing and tail-flick with P<0.01 and chronic phase of formalin test (P<0.001). Results showed there were no significant difference between the morphine and HEEA (200 mg/kg) groups in the chronic phase of formalin. Using naloxone+ HEEA at dose of 300 mg/kg has been reversed analgesic activity in all three assessment pain tests. Injection of different doses of extract up to 5000 mg/kg has been not revealed acute toxicity.

Conclusion: The Erigeron acer has central and peripheral analgesic effects. This analgesic effect could possibly be applied through of opioid mechanism.