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Effects of the four drug conditions (caff/caff, caff/plac, plac/plac, plac/caff) on two illustrative subjective measures that did not show withdrawal effects but did show acute caffeine effects and tolerance to caffeine. Brackets indicate significant effects. Acute caffeine effects were demonstrated by a significant difference between the plac/caff and plac/plac conditions (a). Tolerance was demonstrated by a significant difference between the plac/caff and caff/caff conditions (b).
