Energy drink manufacturers are exploiting a legal loophole and selling over-the-counter products with unsafe levels of caffeine.

The drink-makers are governed by Food Standard Authority regulations that mandate a maximum level of caffeine of 320mg per litre.

However, they skirt this regulation by repacking their products as "energy shots", says public health nutritionist Bronwen King.

"Those limits apply to food, but they get around that by calling it a dietary supplement," she said.

Energy drink insider Chris Newey said the dietary supplement label was effectively a backdoor way of bringing the shots to market. He said the law surrounding dietary supplements was vague.

"There's a lot of room in the dietary supplement area for a lot of quackery to go on," he said.

A Herald on Sunday investigation found that the Demon Energy Shot contained 3333mg of caffeine per litre - more than 10 times the limit.

Demon Drinks also makes the NOS energy drink, which contains 480mg of caffeine per litre.

The company did not respond to repeated requests for comment.

Five other energy shots were examined and all contained more than 320mg of caffeine per litre.

Nutritionist Stacey Hancock said the energy shots were potentially dangerous.

"They can have a lasting effect on the central nervous system and stimulate the adrenalin gland, which increases the body's stress levels," she said.

"They can lead to type 2 diabetes and it's possible that it raises blood pressure."

Auckland mother Brooke Robertson suffered a heart attack this year and attributes her brush with death to her diet of 10 to 14 cans of Red Bull a day.

A 15-year-old Wellington girl collapsed in August after drinking a high-energy shot bought from a dairy near her school.

King was concerned that energy shots were being marketed to children, despite these risks.

"If you look at the bottom of the bottle, in small print, they say 'not recommended for children'. But often they're placed next to the lollies."

The Food Safety Authority is investigating the legality of labelling energy shots as dietary supplements, and Newey said it needed to act fast to close the loophole.