Alcohol is one of the world's favorite recreational drugs. After a drink, most people become more sociable, less stressed, and happier, and will probably reach for another drink before too long. For all this to occur, the alcohol in the drink has to do a multitude of things.
When you sip that tasty gin and tonic, the ethanol goes to your stomach and small intestine, where it is rapidly absorbed into your bloodstream. Some of the ethanol gets broken down in the liver to give you energy (about 7 kcal per gram), and some of it gets to the brain.
Once in the brain, it starts interfering with various kinds of neurotransmitters—the messenger molecules that tell different parts of the brain to start or stop various activities. Though we don't know what exactly happens, we have some clues.
On your behavior
Alcohol is thought to affect two of the brain's most important neurotransmitters: glutamate, which stimulates brain electrical activity, and GABA, which inhibits it. Alcohol blocks the effects of glutamate and enhances the effects of GABA, so the overall result is that alcohol acts as a depressant, making you more sociable and relaxed.
Because of its sedative effects, alcohol makes falling asleep easier. However, it reduces the quality of sleep, because it decreases the amounts of two of the three phases of sleep—the slow-wave and rapid-eye-movement (REM) phases. REM sleep is also to critical to memory formation, and it is one reason why drinking too much is associated with blackouts.
Drinking can also numb pain—not just emotional distress, but actual physical pain. This it achieves by dampening down the pain signals that sensory neurons send to the brain. But this effect is highly variable and doesn't happen in everyone.
The alcoholic "high"
Alcohol isn't just a depressant. It also stimulates the production of dopamine, the chemical associated with many pleasurable activities such as sex, good food, and playing video games. Dopamine is a key part of the reward-motivation system in the brain. The more dopamine an activity releases, the more likely you are to engage in that activity for another shot of dopamine release.
The warm glow
Beyond the mind, alcohol has other physical effects. Though you feel a certain amount of warmth after a drink—sometimes called a "beer jacket"—you in fact get colder faster.
This is because alcohol messes with the hypothalamus, the part of the brain that, among other things, regulates body temperature. Normally, when you feel cold, your body reduces blood flow to your skin and directs it to the organs, to preserve your core temperature. Alcohol reverses that reflex (paywall), sending blood to the skin, which makes you feel warm. But it also means more heat is leaking from your warm blood to the outside world, so if you're not dressed warmly enough you can be at risk of hypothermia.
When it comes to sex, men and women respond differently to alcohol. For men, in general, alcohol reduces both arousal and pleasure. For women, sexual arousal goes up but pleasure goes down. (I found one study supporting the "beer goggles" hypothesis, which says alcohol lowers sexual inhibition by making other people look more attractive to you, but it uses only a small sample of subjects.)
All these effects vary from person to person based on factors such as genetics, body size, and meal times. But when you have too much to drink, all the effects become severe. In extreme conditions, lower glutamate and higher GABA levels are what cause slurred speech, uncoordinated movement, and difficulty putting thoughts together. Consistently drinking too much damages development of the brain, harms the heart, and increases the risk of cancer. More than one in 20 deaths in the world every year are caused because of alcohol consumption. So drink safely.
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