Formalin test in mice, a useful technique for evaluating mild analgesics.

A modification of the formalin test appropriate for testing of mice is described. Formalin 1 or 5% was injected into the dorsal surface of a hindpaw, and the time the animal spent licking the paw was recorded. On the basis of the response pattern, two distinct periods of intensive licking activity were identified; an early (0-5 min after injection) and a late response (20-30 min after injection). The following analgesics were investigated (dose range): acetylsalicylic acid (100-400 mg/kg), paracetamol (100-400 mg/kg) and morphine (0.6-10 mg/kg). Acetylsalicylic acid (200-400 mg/kg early response, 300-400 mg/kg late response), paracetamol (200-400 mg/kg early response, 300-400 mg/kg late response) and morphine (2.5-10 mg/kg) inhibited the responses in a dose-dependent manner. The results indicate that the test is useful for evaluating mild analgesics. It may have advantages over some of the tests that are commonly used for testing analgesics.