Caffeine!

... These results show that caffeine can increase the extent to which people systematically process and are influenced by a persuasive communication.

That's from the abstract of the paper on the Effects of caffeine on persuasion and attitude change.

I learnt of this work through Cosma Shalizi's link-ful post, in which he uses the findings of the above study to speculate (jokingly, of course) about the possible connections between coffee-houses and Enlightenment and the 'rise of the public sphere'.