Caffeine is a fascinating drug. From Coffee Impairs Short-Term Memory:
The study divided 32 college students into two groups. One group was given 200mg of caffeine, which is equivalent to two strong cups of coffee, and the other was given a dummy drug. The students were then asked to answer 100 general knowledge questions that had simple, one-word answers.Caffeine aided word recall when the words were similar to the answer. But when the words were unrelated to the answer, the students who had taken caffeine had more trouble recalling the answer than those who had taken the placebo.