Non-steroidal anti-inflammatory drugs (NSAIDs) are widely known as painkillers. The analgesic action of NSAIDs is attributable to the inhibition of prostaglandin synthesis that occurs in response to blocking cyclooxygenase activity. The effective dose of NSAIDs can vary depending on pain intensity and administration timing; however, there are few studies on this. This study aimed to elucidate whether the analgesic effect of NSAIDs changes depending on the situation in which they are taken and we focused on the NSAID, aspirin (ASP). In a rat model of brewer's yeast-induced inflammation, pain caused by 20% (w/v) brewer's yeast-treatment was defined as "strong pain" and that caused by 2.5% (w/v) was defined as "weak pain". The analgesic effect of ASP (low-dose; 44 mg/kg or high-dose; 66 mg/kg) against strong pain was dose-dependent, but that against weak pain was the same. Furthermore, we defined drug administration after 3 h of brewer's yeast-treatment as "late administration" and that after 20 min as "early administration". In the case of strong pain, the analgesic effect of "late ASP administration" was dose-dependent, but that of "early ASP administration" was the same. These results suggest that low-dose NSAIDs have an analgesic effect against weak pain or when taken early.
Keywords: administration timing; analgesic effect; aspirin; inflammatory pain; non-steroidal anti-inflammatory drug; pain intensity.