Introduction: Previous studies have shown the analgesic, anticonvulsant, spasmolytic, and anti-inflammatory effects of Urtica dioica (UD). In the present study the effects of hydroalcoholic extract of UD on morphine withdrawal signs were investigated. Acute toxicity (LD50) of the extract was also assessed. Methods: In an experimental study, 48 male NMRI mice were randomly divided into 6 groups of 8 each, consisting of control (10 mL/kg), clonidine (3.5 mg/kg), and different doses of UD extract (25, 50,100 and 200 mg/kg). Morphine dependency was induced by administration of different doses of morphine (50, 50, 75, and 50 mg/kg) within a four-day schedule (1st-4th day, respectively). On the last day, after administration of a single dose of morphine, naloxone (5 mg/kg) was injected and the withdrawal signs were recorded within 30 minutes. To assess acute toxicity (LD50), 12 extra rats were used and toxic effects of different doses of the extract were evaluated by Lorke’s method. Results: All doses of the UD extract, compared to control group, significantly decreased the number of jumping, grooming, teeth chattering, rearing, wet dog shakes, diarrhea, writing and climbing. In addition, the LD50 of the extract was 2.9 g/kg. Conclusion: UD extract could decrease the morphine withdrawal signs and might be beneficial in addicted patients. However, further studies are needed to clarify the exact mechanism of its action.