Caffeine is a stimulant. It acts on the body's central nervous system within minutes, increasing alertness and reducing sleepiness.

But it has other effects too. It can raise your heart rate and make you feel jittery or anxious.

And once you've drunk it, it will take hours to clear it from your system.

Having a few cups of coffee or other caffeinated drinks a day is considered perfectly safe. But drinking too much or lots in a short space of time is risky.

You can overdose on caffeine and it is possible to die if you ingest too much.

Up to 400mg of caffeine a day appears to be safe for most healthy adults. That's roughly the amount of caffeine in four cups of brewed coffee, 10 cans of Coca-Cola or two "energy shot" drinks (although check the caffeine content of the beverage as it can vary).

Adolescents and pregnant women are advised to have less than this, though. Caffeinated drinks are unsuitable for toddlers and young children.

Most energy drinks contain a caffeine equivalent of three cups of coffee and as much as 14 teaspoons of sugar.

In 2015, the European Food Safety Authority said drinking more than 400mg could lead to increased heart rate, higher blood pressure, irregular heartbeat, tremors, nervousness, insomnia and panic attacks.

Caffeine warning signs

You may want to cut back on caffeine if you experience side effects such as:

- Headache
- Insomnia
- Nervousness
- Irritability
- Restlessness
- Stomach upset
- Fast heartbeat
- Muscle tremors

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