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We all (presumably) love coffee. And we love it, if not for its flavour, for the effect it has on us. Caffeine is the most widely consumed psychoactive drug in the world, and while it's present in a lot of different foods and drinks, like tea, chocolate or guarana, the vast majority is consumed as coffee.  
But what is it? What’s going on when we take it? How much is too much?  

The Chemistry

Caffeine is an alkaloid, which is a type of organic compound that contains at least one nitrogen atom.

Common among alkaloids is a bitter flavour. The primary function for caffeine in plants is as a pesticide, deterring insects and animals from eating the plant. They achieve this through the bitter flavour, but it is also toxic to many insects. Indeed, this is where our instinctive dislike of bitter tastes stem from: it often means poison. 

How it works

Caffeine affects the body with three different mechanisms: Most simply, caffeine increases the body’s production of dopamine and noradrenaline. 

But it also blocks the body’s adenosine receptors, which, when triggered with adenosine, make you feel drowsy. Blocking the receptors prevents this from happening, making you feel more awake. This is also probably why many people experience a caffeine slump a few hours after drinking coffee: after the caffeine stops blocking the receptors, a build-up of adenosine quickly fills the receptors and sends you into a sleepy daze. 


As a psychoactive drug, caffeine is highly addictive. You probably already knew that. This addiction, and the withdrawal you feel in the morning, quickly becomes the primary thing that you are actually craving caffeine for. While the stimulating effects are very real, it generally takes about 30 minutes for caffeine to be properly absorbed into your system and start having the energising effects. The initial feeling of awakening you get almost immediately after your first sip of coffee is actually just the withdrawal symptoms receding. 

You’ve probably been stuck without coffee in the morning before, something that, if you’re a heavy coffee drinker, is a highly unpleasant experience. Symptoms include headaches and irritability, and generally take a full day to dissipate if you miss out on your daily caffeine dose. Hitting these symptoms on the head is, at least first thing in the morning, the primary function that caffeine is serving in heavy coffee drinkers. 

How much is safe?

Unfortunately, it’s very hard to measure how much caffeine is in a particular cup of coffee. 

Fundamentally it is measured in the same way as in alcohol: by  percentage. A bottle a wine will tell you how much alcohol is in the bottle – normally about 10-14% – and this gives you an indication of how strong the effects of alcohol will be. No such number is written on the side of your coffee, not even if you bought it in a café. 

Not only does the caffeine content vary between varieties, but it also varies depending in how you brew it. A high-dose cup of filter coffee has far more caffeine than a traditional low-dose espresso, even if you brew them with the same coffee.  

Moreover, the ability to process caffeine varies from person to person, not just by their body weight but also by the speed of their metabolism. 

So, it’s difficult to say exactly how many cups of coffee are safe to drink in a day. Recommendations say no more than 400mg of caffeine per day for an adult. Some researchers have seen anything from around 60mg up to 300mg of caffeine in cups of coffee from various cafés. 

Luckily, for caffeine intoxication to be fatal, you’d have to ingest around 10g – 10 000 mg – of caffeine. You’ll have a severe headache and feel absolutely awful from the effects of over-caffeination well before you reach that. But that’s not a free pass to have as much coffee as you want: keep an eye on your consumption and how you respond to coffee.