The ultimate guide to caffeine

Coffee beans lying on a surface.

Caffeine is a very important drug for everyone. I’m sure most people appreciate the effect that a cup of coffee or tea has on them, or notice the buzz when they inject it straight into their veins – probably best not to do that.

The attention improvement that it gives you, can turn an unproductive day around, and can allow you to work for many hours straight, but… what is caffeine? …and what does it actually do to you? If you drink too much, do you overdose? I’ll be trying to answer these questions and more in this post, so read on if you want to know more about the drug that you consistently take. If you don’t drink caffeine, you can still learn about it and tell all your caffeine drinking friends how lucky they are (or unlucky – I won’t spoil it yet – maybe it is bad?).

What is caffeine?

Caffeine chemical formula

1,3,7-trimethylxanthine, C8H10N4O2

Now… unless you are an organic chemist or a genius, that probably doesn’t mean much to you. Fear not, there are many dedicated people who have researched its effects for us, so we don’t have to strain our minds too much. You can skip the scientific background if you just want to get straight to the effects, or just find out what you can drink to get more caffeine and make the most out of this natural ‘high’.

Caffeine is an alkaloid, which is a group of commonly psychoactive compounds that contain at least one nitrogen atom. These compounds generally don’t have colour and are bitter [1]. Morphine and nicotine are examples of other compounds from this group, however I think you’d be more cautious about ingesting these regularly due to their more intense and addictive nature. That is not to say that caffeine can not be addictive because it can, and in high quantities it can still be dangerous. In a cup of coffee, you are likely to be consuming about 100 mg of caffeine, which is a reasonable dosage with noticeable psychological and physiological effects.

Caffeine can be found in many natural plants including tea, coffee, guarana, maté, kola nuts, and cacao. Many forms of caffeine have been used throughout history for their medical effects.

A short history of caffeine.

Caffeine has been around for a long time and has been consumed in many forms. It is a natural product from plants. The origin of the English word caffeine likely comes from the French word caféine. Throughout human history, caffeine has been used in many ways.

One common form of caffeine consumption is tea. Legend suggests that tea was first discovered in China by Emperor Shen Nong in around 2700BC when a leaf off a shrub fell into boiling water. Tea is of large importance to the history and culture of China. Tea was much later imported into Britain in the 1600s when Dutch traders brought it into Europe. Since then, it has been a common beverage, so much so that it is one of the most common drinks in Britain. The Independent states the average Brit gets through 1460 tea bags a year – which is… a lot.

Another form of caffeine is coffee. This is suggested to trace its origin to the Ethiopian plains and then cultivation on the Arabian Peninsula. In around the 17th century it reached Britain and the rest of Europe. Interestingly, tea was originally the favoured drink in the ‘New World’. However, when the British increased tax dramatically on tea, in the 1773 ‘Tea Act’, many people resisted the taxes and started to drink coffee as an alternative. This increased the popularity and consumption of coffee massively.

The first time the chemical form of caffeine was discovered and isolated was 1819 by Friedlieb Ferdinand Runge. Since then, it has been the focus of a large amount of research, due to its importance and how widely used it is in modern society. It is the most used drug in the alkaloid family, so understanding its effects is of upmost importance.

Caffeine in medicine

Caffeine is a very popular recreational drug, but interestingly, caffeine is also used in medicine. In combination with painkillers like aspirin, it can reduce migraines.

An assortment of pills.
An assortment of pills – Photo by Pixabay on

It can also be used for “Neonatal apnoea”. This is a condition where newborns have pauses in breathing causing drops in heart rate and low oxygen levels. This is common with premature births. The form of caffeine-citrate is approved for medical use in this instance. Caffeine doesn’t prevent this condition from developing [2].


Disclaimer: although I have read some research papers on caffeine I am by no means a expert, so avoid using this for medical advice.

Toxicity Thresholds – aim not to drink more then this:

19 years or older: ~400 mg/day
12-18 years or older: ~100 mg/day
<12 years old: 2.5 mg/kg/day
Pregnancy: No more than 200 mg a day as caffeine has been known to cause complications.
The European Food and Safety Authority considers 3 mg/kg/day of habitual caffeine consumption safe for children and adolescents.


Enhances cognitive functioning

There are many cognitive benefits to consuming caffeine. One paper found that caffeine reduces reaction time on tasks that relating to attention and working memory. It also found that caffeine improves sentence verification accuracy [3].

Top tip: Caffeine is a great tool for students when revising gets hard. Just don’t drink too much.

Increases endurance and enhances performance

There are many scientific studies that demonstrate that caffeine is good for performance. Aerobic exercise is the form of exercise with the most consistent benefits from caffeine consumption. As with many drugs, the effects vary from person to person, so it’s up to you to work out whether it is helpful [4].

Boosts metabolism

Research suggests that the consumption of caffeine can increase daily energy expenditure by around 4%. This is definitely a reasonable increase [5]


Some negative effects of caffeine are listed below. The effects of Caffeine differ for each person. If by consuming caffeine you feel serious negative effects, stop consuming it immediately. If the effects continue after stopping, make sure to get in contact with a professional.

Increases anxiety

Caffeine can increase your natural anxiety. It is advisable not to drink caffeine before anxiety provoking situations. I tend to avoid it on days when I am doing things that I tend to get anxious about, but then I am anxiety prone.

Inhibits sleeps

Although sometimes you may want to use caffeine because it keeps you awake, it can also be seen as a negative effect. It is advisable not to consume high quantities of caffeine around 6 hours or less before sleep since it will not be out of your system. The half life of caffeine is around ~4-5 hours.


You can unfortunately get addicted to caffeine. Whether a small addiction to caffeine is bad for you is questionable, but if you start drinking over 400 mg regularly each day, it is likely that you should try to reduce or cut caffeine out of your diet.


In very high doses it can cause digestive issues. In smaller doses it can stimulate your digestive system through production of the hormone Gastrin and therefore cause a laxative effect [6].


You will need to go to the toilet more when drinking reasonable amounts of caffeine. If you want to hydrate, pick water or a drink that isn’t a diuretic. The reason that caffeine is a diuretic is that it inhibits some kidney and liver functions, increasing diuresis and natriuresis [7].

Top tip: don’t consume caffeine if you expect to be very far from a toilet.

Consuming Caffeine

Drink Options

The caffeine-o-meter, categorizing roughly how good the source of caffeine is for you. These ratings are for a bit of fun, so take them with a pinch of salt.

?⚫⚫⚫⚫1/5 – this is a bad source of caffeine, a bad buzz.
??⚫⚫⚫2/5 – alright but really you should find something else.
???⚫⚫3/5 – getting average here, this is a reasonable buzz.
????⚫4/5 – a good source of caffeine, making good choices.
?????5/5 – perfect source of caffeine. I can feel the buzz through my screen.

Coffee ~ 90+mg | ????⚫

Just the classic source of caffeine. A good drink for any occasion. You will definitely feel the effects of this (I say this while knowing someone who doesn’t). Make sure to get a great flavour and this will be a great start to your morning. If you are drinking the coffee from somewhere like Starbucks, it can have significantly higher caffeine quantity, so watch out.

Tea ~ 30+mg | ????⚫

Another classic form of caffeine. Though it is a less ‘popular’ form of caffeine – not a less popular drink – the kick will definitely wake you up in the morning. This is one of my go to choices for the morning wake up. It’s good for a bit less of an intense morning.

Green Tea ~ 30+mg | ?????

Less popular but a great source of caffeine. If you want a rich flavour, this probably isn’t what you want. Not only does it give you a caffeine high, it also has L-theanine in it which can improve some effects of caffeine, and can make the caffeine ‘kick’ last longer in a more ‘focused’ fashion.

Top tip: Green Tea is one of the best sources of caffeine.

Energy drink ~ 90+mg | ??⚫⚫⚫

Not so good. You’ll definitely get the kick out of this but the amount of sugar and other things removes all the goodness that caffeine gives you. This definitely won’t help you get the 10% smaller chance of dying. However, if you are just here for the kick, drink on!

Hot chocolate ~ 5+mg | ?⚫⚫⚫⚫

I rated this a 1/5 because if you want a caffeine kick, this is not the way forwards. You’ll have to drink 18 of these to even get the caffeine content of one cup of coffee, and by then I’m pretty sure you will be curled up on your bed, not wanting to move for a very long time. However, if you just want to relax, go make yourself one and put on some music.

Food (Solid) Options


The caffeine content of chocolate varies based on the percentage of cacao solids however for a 100g serving a common range of caffeine content is 20-40 mg depending on the cacao content and the brand [10].


Another form of solid caffeine is supplements. Many supplements contain caffeine, though the benefit or necessity for caffeine supplementing is uncertain and currently a topic of research.

How does it work?

Caffeine is an adenosine inhibitor. This means blocks adenosine binding to the adenosine receptors by binding itself to the adenosine receptors instead. Adenosine is a natural neurotransmitter in the body (released through the use of energy), which would normally bind to these adenosine receptors and increase the bodies natural feeling of tiredness. Taking caffeine will inhibit this pathway meaning you feel more awake. After the level of caffeine in the body reduces these receptors will be bound to by the adenosine once more and the feeling of tiredness will set in.


[1] “Caffeine.” Britannica Academic, Encyclopædia Britannica, 29 Mar. 2016. Accessed 20 May. 2021.

[2] Caffeine for medical usage

[3] Haskell, C.F., Kennedy, D.O., Wesnes, K.A. et al. Cognitive and mood improvements of caffeine in habitual consumers and habitual non-consumers of caffeine. Psychopharmacology 179, 813–825 (2005).

[4] Guest, N.S., VanDusseldorp, T.A., Nelson, M.T. et al. International society of sports nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr 18, 1 (2021).

[5] Caffeine consumption and energy expenditure

[6] Caffeine Effects on Digestion

[7] Mécanismes de l’effet diurétique de la caféine Barbara Marx, Éléonore Scuvée, Jacqueline Scuvée-Moreau, Vincent Seutin and François JouretMed Sci (Paris), 32 5 (2016) 485-490

[8] Caffeine Chemical

[9] Fredholm BB. Notes on the history of caffeine use. Handb Exp Pharmacol. 2011;(200):1-9.

[10] Caffeine content in chocolate

[11] Haskell CF, Kennedy DO, Milne AL, Wesnes KA, Scholey AB. The effects of L-theanine, caffeine and their combination on cognition and mood. Biol Psychol. 2008 Feb;77(2):113-22.

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