Caffeine is the most widely used cognitive enhancer in the world. It blocks adenosine signaling, which can reduce sleepiness and improve alertness. It may help reaction time, vigilance, and perceived energy, but too much can cause jitteriness, anxiety, or sleep disruption.

L-theanine is an amino acid commonly found in tea. People often combine it with caffeine because it may promote a calmer, smoother feeling while preserving alertness. The pair is popular because it can reduce some of caffeine's rough edges for certain people.

Nicotine can increase alertness and attention, but it carries meaningful risks, including dependence. For that reason, it is not a casual performance tool. A substance can have cognitive effects and still be a poor choice overall if the harms are large.