Fitness fanatics have been eating powder supplements neat, rather than diluting them in water to make a drink as recommended by manufacturers.
A US medical conference heard youngsters may try it as seen on by viral social media videos.
They counted 30 videos which had gained eight millions likes on TikTok.
Pre-workout powders typically contain amino acids, vitamins and caffeine to boost workout stamina. There are risks with taking on too many energy-boosting stimulants.
Large doses of caffeine can cause heart palpitations, extra or missed beats.
A scoop of powder could be the equivalent of caffeine as five cups of coffee, the Cohen Children’s Medical Center researchers from New York have said.
In the UK, such products are regulated as foods rather than medicines. It is considered safe for sale in shops to over-18s.
It is feared powders sold online may not be from reputable suppliers or may not even contain the ingredients it says it does.
The study researchers looked at 100 TikTok videos using the hashtag “preworkout” and only eight of them showed the powder being used in the correct way.
More than 30 of the videos showed individuals dry scooping. These videos attracted more than eight million likes.
The researchers issued a warning in their presentation for the American Academy of Pediatrics meeting.
They said: “Physicians should be aware of the pervasiveness of pre-workout, dangerous methods of consumption, and the potential for accidental over-consumption, inhalation, and injury.”
Nutrition scientist Bridget Benelam, from the British Nutrition Foundation, told the BBC: “Pre-workout powders typically contain caffeine along with other ingredients such as creatine, amino acids and vitamins.
“There doesn’t appear to be much research on the benefits of these products, although there is some evidence that caffeine may improve sports performance in some cases. These studies are typically done in athletes, and so it’s not clear how relevant this is for the wider population.
“The levels of caffeine in these products vary from the equivalent of about one to over three cups of filter coffee, if made up according to the manufacturer’s instructions.
“So, there is a risk of over-consuming caffeine, especially if using more than once a day, or just consuming the powder, where you may consume more than the recommended amount.”
The British Heart Foundation advises gym-goers should have six to eight glasses of fluid a day whether training or not.