Caffeine is a stimulant that can make you feel more alert and less sleepy due to its effect on the brain and nervous system.
Lewis James, from the School of Sport, Exercise and Health Sciences at Loughborough University, says there is good evidence that caffeine can help make exercise feel easier.
It's become one of the most regularly used supplements among athletes.
As you exercise your body produces a chemical called adenosine which makes you feel tired. Caffeine blocks the adenosine receptors in your nerves, so your brain perceives less pain and fatigue.
But it acts on other parts of the body too - including the cardiovascular system - which can be risky.
High levels can cause a rapid heart rate, abnormal heart rhythms and seizures.
Although rare, there are documented deaths from excess caffeine.
Some people are more sensitive to caffeine than others and can feel nauseous, anxious and irritable, and get headaches even at lower doses.
Generally, up to 400mg of caffeine a day appears to be safe for most healthy adults - that's around four cups of instant coffee.
Tea contains a bit less, so five cups a day is usually fine.
Pregnant women are advised to halve daily intake to 200mg or less.
Children and teenagers are also more susceptible to the risks and potential for an overdose.
This is why energy drinks containing over 150mg of caffeine, for example, are already required by EU law to carry labels stating: "High caffeine content. Not recommended for children or pregnant or breast-feeding women".
