Caffeine is the world’s most most widely consumed psychoactive drug.

Weird facts about caffeine

1. Unlike other stimulants, caffeine works by antagonizing adenosine receptors, rather than by acting directly on monoamines.
2. Caffeine consumption reduces the risk of cancer, notably liver and cervical cancer.
3. Yet it may increase the risk of bladder cancer.
4. It also reduces the risk of cardiovascular disease, type 2 diabetes, and high altitude sickness.
5. Contrary to a popular myth, caffeine is not a diuretic. (The increased urination noted in those old studies was due to the temperature of the liquid, not its caffeine content.)

Mechanism of action

When well-rested and awake, there is little adenosine present in your CNS neurons. As you stay awake, adenosine accumulates in neuronal synapses and eventually finds its way to CNS neurons with adenosine receptors, which then produce the cellular response that we experience as drowsiness.

Caffeine is an adenosine receptor antagonist and effectively prevents the onset of drowsiness induced by adenosine. Its mechanisms of action:

1. Antagonism of adenosine receptors by caffeine stimulates the medullary vagal, vasomotor, and respiratory centers, which increases respiratory rate, reduces heart rate, and constricts blood vessels.
2. Adenosine receptor antagonism also promotes neurotransmitter release (e.g., monoamines and acetylcholine), which induces CNS arousal. The key monamines here are epinephrine and norepinephrine, the end products of the catecholamine synthesis pathway: phenylalanine → tyrosine → dihydrophenylalanine (DOPA) → dopamine → norepinephrine (noradrenaline) → epinephrine (adrenaline). (See diagram below).
3. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine.
4. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons, which increases in locomotor activity in animals. (The effects of caffeine on learning, memory, performance and coordination are actually due to the methylxanthine action on arousal, vigilance and fatigue.)