Caffeine crystals

What’s wrong with too much caffeine?

Many of us reach for a cup of tea or coffee when we’re tired, but why? Jennifer Trent Staves finds out how caffeine works and why consuming too much can be bad for you

What is caffeine?

An odourless white powder that is bitter in taste, caffeine is a chemical with the formula C8H10N4O2. This stimulant is found naturally in over 60 plant species – including coffee, tea, kola nut and cocoa – so its presence in our cups of tea and coffee and bars of chocolate is unsurprising. However, its stimulative power has led to manufacturers adding synthetic forms of caffeine to other products, such as fizzy soft drinks.

How does it work?

Caffeine works by competing with adenosine, a chemical produced by the brain. Normally, adenosine binds to receptors to slow down nerve cell activity, but caffeine takes adenosine’s place, preventing it from doing its job at these locations in the brain. As a result, the activity of these nerve cells increases. This affects several parts of the body, including our central nervous system, the kidneys, muscles and the cardiovascular system. Caffeine starts working as soon as 15 minutes after consumption. It takes about six hours for half of the total caffeine consumed to be eliminated from the body.


  • increases heart rate
  • constricts blood vessels
  • has a diuretic effect (increases the amount of urine made)
  • relaxes air passages
  • encourages the production of adrenaline (because of increased nerve cell activity)
  • contracts muscles
  • suppresses appetite.

What are the positive effects of caffeine?

Caffeine is a stimulant, so it can increase alertness. By encouraging the production of adrenaline (which is linked to our fight-or-flight reflex), caffeine can improve productivity and concentration.

Modern medicine has harnessed caffeine’s power in a variety of ways. Caffeine is often included in paracetamol tablets and other cold remedies, as it can help treat headaches by constricting blood vessels.

Because it can relax air passages, caffeine has been used to treat premature babies with a condition called sleep apnoea, which causes them to stop breathing in their sleep. One Australian study found caffeine to be effective in reducing the number of attacks in the first week of treatment and associated it with better longer term outcomes as well.

What are the negative effects of caffeine?

A dose of caffeine in a morning cup of tea might help us work, but many of the positive effects of the drug become negative ones when the amount of caffeine consumed is increased. Like many drugs, caffeine can cause physical dependence.

Too much caffeine is associated with reduced coordination, insomnia, headaches, nervousness and dizziness. Cutting caffeine out completely (going ‘cold turkey’) can cause withdrawal symptoms, such as headache, fatigue and muscle pain.

Ingesting excessive amounts can also put a strain on the heart and is linked with increased blood pressure and raised blood cholesterol.

Should we limit our consumption?

Europeans are the largest consumers of caffeine: a 1999 study into the consumption of coffee, tea, mate (a South American infused drink) and cocoa identified the UK as the world’s 15th largest consumers of caffeine, after some other European countries and Australia, Nicaragua and Canada.

An older study on caffeine intake found that the average daily intake among UK adults was 359 mg, or 5.1 mg/kg for a 70 kg person. Another study estimated it to be 4 mg/kg for all consumers of caffeine and 7.5 mg/kg for heavier consumers.

In excessive doses, caffeine can even be lethal: the fatal dose is more than 10 g (80–100 cups of coffee in rapid succession), according to the University of Washington.

Although drinking 100 cups of coffee in one go is highly unlikely, studies say the amount of caffeine consumed in drinking four cups of coffee (or two small energy drinks) a day can cause physical dependence and have unwanted side-effects in adults.

Some groups are more at risk and should limit their intake even further, including:

Pregnant women or nursing mothers

The NHS advises that pregnant women should have no more than 200 mg of caffeine a day – this was reduced from 300 mg in 2008. This advice stems from research associating caffeine with lower birth weight in babies, as well as spontaneous miscarriage. Although there is no set guidance for nursing (breastfeeding) mothers, caffeine intake should be limited because it readily passes through breastmilk.

Babies and children

Caffeine contains a specific type of chemical (polyphenols) that bind to iron, making it difficult for the body to absorb this element. Because iron is essential for growth and development in young children, too much caffeine can stunt development. The drug has also been blamed for sleep problems in young children. A 2010 study by Dr Warsak and colleagues at the University of Nebraska Medical Centre found that the more caffeine children consumed, the less they slept.


Older studies have found that teenagers in the UK consume more caffeine than their counterparts in the USA and Australia. Drinking too much caffeine can affect sleeping patterns in teenagers – a concern for students who stay up late at night and fall asleep during the school day.

In a 2009 study, researchers found that caffeine consumption tended to be more than 75 per cent higher among those teenagers who struggled to stay awake during the day than among those who didn't struggle. The researchers suggested that teenagers limit their caffeine intake to the first part of the day so as not to interfere with night-time sleep.

Amount of caffeine in everyday food and drinks

Coffee: 60–150 mg in 150 ml (a small cup)

Decaffeinated coffee: 2–5 mg in 150 ml (a small cup)

Tea: 40–80 mg in 150 ml (a small cup) 

Coca-cola: 4 mg in 330 ml (one can)

Energy drinks: 80 mg in 250 ml (one can)

Chocolate: 3–63 mg in 50 g (one bar)

Weight-loss tablets: 16–200 mg in one tablet

Source: University of Washington

Lead image:

False-coloured scanning electron micrograph of caffeine crystals. 

Annie Cavanagh and David McCarthy/Wellcome Images


Further reading

About this resource

This resource was first published in ‘Food and Diet’ in June 2011 and reviewed and updated in August 2016.

Microbiology, Medicine, Health, infection and disease
Food and Diet
Education levels:
16–19, Continuing professional development