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A small can contains the same amount of caffeine as a regular coffee. But young people seem attracted to the fizzy alternatives. In the battle to win sales, makers are introducing new flavours, super-sized cans and higher concentrations of caffeine to give drinkers a bigger hit.

We wanted to know how easy it is for children to buy these drinks, so we sent Anna - who's 15 - on a shopping spree to the main supermarkets to find out. She met X-Ray presenter Rhodri Owen, who was shocked to discover the number of cans she was able to purchase.

Anna told Rhodri: "I could just buy them, and buy a lot of them, so easily. It does surprise me a lot, and anyone could buy them, people younger than me."

Legally, any drink with more than 150 milligrams of caffeine per litre must tell the customers on the packaging that there's a "high caffeine content" - and they must show the amount of caffeine per 100 millilitres.

According to the Food Standards Agency, that will "alert consumers to unexpectedly high levels of caffeine in some drinks".

And some manufacturers go even further, all the cans that Anna bought carry messages that the drinks aren't suitable for children, but that doesn't seem to stop children drinking them.

There's been a spate of incidents over the last few years where children have needed medical attention after drinking too many high caffeine drinks. Some people think there should be more restrictions on how they're sold.

Mum of three and dietician Chris Cashin is so concerned she doesn't want them sold to children at all.

She explained: "From my own experience with various sports, the drinks are left at various venues - at athletics venues, rugby, football, cricket, so the drinks are really widely available, and children see them there and you see the signs everywhere.

"Most children, when they drink too much caffeine, become very hyper and they lose concentration, but certainly when they play sport, it makes them very aggressive to start with and they suddenly become very quiet because the effects give them quite a down; and so they then can't perform and you often find they have to stop playing."

According to the Food Standards Agency, there's no recommended safe limit on how much caffeine young people should be allowed to drink, because sensitivity to caffeine differs from person to person.

But the FSA admits that drinking just two small cans, or one large can, could result in "behavioural changes, such as... nervousness or anxiety" in a child weighing 30kg - which is about average for an eight or nine year old.

Caffeine is a drug with a whole range of side effects. Caffeine poisoning can be serious, affecting the brain and major organs. Dr David Bailey from Trethomas has seen patients who've needed hospital treatment after overdosing on caffeine.

He described the symptoms of a caffeine overdose: "You're going to start to get more anxious, you might be shaky, sweaty. You'll feel just very tense, a bit hyper if you like, pulse rate might go up and you might feel that you're having a palpitation.

"You could mix it up with a panic attack, you can mix it up with just general anxiety and some people actually mix it up with thinking it might be a heart attack."

According to Dr Bailey overdosing on caffeine drinks is "pretty much as easy as if you drank a lot of coffee." And the risk is greater for children because "if you're that much smaller then the same amount of caffeine is going to affect you a lot more."

Someone who's well aware of the effects of caffeine in young people is Ian Whittaker, a teacher at Willows High School in Cardiff.

He told Rhodri: "We find that pupils who consume these drinks have a higher state of agitation. They have problems with their concentration.

"We find issues with behaviour, particularly in the afternoon, and as a school we're concerned about the level of caffeine that pupils are consuming.

"We've had significant issues with pupils consuming these drinks particularly at lunchtimes. For example we've had pupils who've come in, openly challenge teachers, disrupt other pupils' learning and are verbally aggressive towards other members of staff and other pupils in the school."

Barry Evans from Drug Education UK ran a session for 15 year pupils at the school to help them understand the effects these drinks can have.

He says: "When we first said, did you realise caffeine is classed as a drug?, basically all the boys and girls there said it's more energy. I think perhaps it's better to educate them earlier on about caffeine.

"They were surprised to understand what is a safe limit, as far as caffeine is concerned, and one boy's friend was drinking in excess of almost a daily amount in one hour, which is worrying."

Despite the warnings, it's perfectly within the law for shops to sell these drinks to children. But what does Dr Bailey make of the amount of caffeine 15 year old Anna was able to buy?

"You're going to be buzzing if you drunk those, it's going to be sending your heart rate through the roof. If you get a very fast heart rate it can potentially be fatal.

"I think that would be unlikely, but that's the sort of potential risk you're talking about and that's the reason the warnings are printed on the can, because caffeine is not a safe drug in larger amounts and it's not safe to drink too much of it."

X-Ray contacted the British Soft Drinks Association which represents some manufacturers. They point out that all their members do label their high caffeine products as "not recommended for children, pregnant woman and those sensitive to caffeine".

They say that these drinks can be consumed in moderation as part of a balanced diet and active lifestyle.

The drinks' makers tell us they don't target children, but only Red Bull was prepared to comment on whether there should be tighter controls on children buying the drinks.

They say "it is difficult to see what could be put into effect that wouldn't also apply to coffee and other caffeinated drinks."

We'd like to know what you think about this. Have you noticed your children's behaviour has changed after they've had these drinks? Contact X-Ray with your experiences.