Caffeine, Alcohol, Nicotine and the Brain

What the world's most common psychoactive substances actually do: caffeine blocking adenosine, alcohol boosting GABA and silencing memory, nicotine hijacking acetylcholine — plus tolerance, sleep and long-term effects.

Caffeine

Caffeine is the world's most widely used psychoactive substance. During waking hours, the molecule adenosine builds up in the brain as a by-product of energy use and promotes sleepiness by acting on adenosine receptors. Caffeine is an antagonist at those receptors: it blocks them without activating them, masking the sleepiness signal and indirectly boosting dopamine and noradrenaline activity. Effects begin within about 15–45 minutes. Caffeine's half-life is roughly 5 hours on average (ranging widely, longer in pregnancy and shorter in smokers), so an afternoon coffee can still affect sleep at night, reducing deep sleep even if you fall asleep fine. Regular users develop tolerance, and stopping abruptly can cause withdrawal headaches and fatigue for a few days. Moderate intake (up to about 400 mg a day for most healthy adults, around four cups of brewed coffee, according to bodies such as the US FDA) is generally considered safe.

Alcohol

Alcohol (ethanol) affects many targets. It enhances GABA-A receptors (more braking), inhibits NMDA glutamate receptors (less excitation), and releases dopamine and endorphins (reward). Low doses can reduce inhibition; higher doses impair coordination via the cerebellum (hence wobbly walking), judgement via the prefrontal cortex, and breathing in severe overdose. Blackouts occur because alcohol disrupts the hippocampus's ability to form new memories — the person is awake and acting, but the brain isn't recording. Alcohol helps people fall asleep but fragments sleep and suppresses REM sleep later in the night.

Long-term heavy drinking damages the brain: shrinkage, white matter damage, and thiamine (vitamin B1) deficiency causing Wernicke–Korsakoff syndrome, with confusion and severe memory loss. Alcohol during pregnancy can cause fetal alcohol spectrum disorders. Recent large studies suggest there is no level of drinking that benefits the brain, and risks rise with amount.

Nicotine

Nicotine activates nicotinic acetylcholine receptors, boosting attention briefly and triggering dopamine release in the reward pathway. It reaches the brain within seconds of inhaling, making it highly addictive. Smoking harms the brain mainly through blood vessels — raising stroke and dementia risk. Effective help includes nicotine replacement, varenicline and counselling.