Human study with low dose of caffeine and moderate dose of aspirin.
http://www.ncbi.nlm.nih.gov/pubmed/3680601
"...We observed that caffeine (64 mg), when added to aspirin (800 mg), significantly improves vigilance performance and increases self-reported efficiency when compared with either placebo or aspirin alone. As previously reported, this caffeine dose alone significantly increased vigilance and decreased reaction time. No adverse effects of caffeine were detected on any of the parameters that were assessed. This study therefore demonstrated that the addition of caffeine to aspirin, in a dose commonly employed in over-the-counter drugs, has significant beneficial consequences with respect to mood and performance."
http://www.ncbi.nlm.nih.gov/pubmed/3680601
"...We observed that caffeine (64 mg), when added to aspirin (800 mg), significantly improves vigilance performance and increases self-reported efficiency when compared with either placebo or aspirin alone. As previously reported, this caffeine dose alone significantly increased vigilance and decreased reaction time. No adverse effects of caffeine were detected on any of the parameters that were assessed. This study therefore demonstrated that the addition of caffeine to aspirin, in a dose commonly employed in over-the-counter drugs, has significant beneficial consequences with respect to mood and performance."