Fig 1. Anti-nociceptive effects of systemic tramadol.

Paw withdrawal thresholds (PWT) were sequentially evaluated before (control) and after the intraperitoneal administration of normal saline (NS), tramadol 0.5 mg/kg (T 0.5), 5 mg/kg (T 5), and naloxone 4 μg/kg (T+N) in this sequential order to the same rats. The PWT was elevated with tramadol 5 mg/kg (n = 6) and was returned to the control level by naloxone. Data are the mean ± SEM. Asterisks indicate a significant difference from the control (*p < 0.05) (one-way analysis of variance and Dunnett’s post hoc test).