Parents are being warned that some slime toys contain high levels of a chemical that can cause convulsions and sickness.
British consumer group Which? tested 11 children's slime products - and found that eight breached EU limits on the amount of boron they contain.
It comes as slime and slime products have become increasingly popular among children, with YouTube tutorials going viral.
The trend has become so popular that Ireland's top 'how-to' Google search last year was 'how to make slime.'
Science journalist Sean Duke says boron can be toxic:
"This is something that is put into slime for stickiness," he said.
"A problem has been found that actually a number of these slime products contain more borax than actually is allowed by EU regulations.
"It can cause dizziness; it can cause the child to be sick, nauseous; it could actually cause more serious effects at a higher level."
Which? warns that exposure to high levels of boron can cause irritation, diarrhoea, vomiting and cramps.
Over the longer period however, studies of boron in animals have been linked to low birth weights, birth defects and developmental delays.
The EU states that exposure to very high levels may impair fertility and cause harm to the unborn child in pregnant women.
All eight of the products found to contain high levels of the chemical were available on Amazon - but have since been removed.
Mr Duke said products purchased on the high street were "generally found to be OK" but some of the one that can be bought online were found to be dangerous.
He said the slime-making process involves mixing a number of different ingredients and warned parents to supervise their children throughout the process.
"Some of these slime products, people just think they are safe, when actually they are a mixture," he said.
"When kids do this, it is like a recipe in a cocktail.
"Don't let them do it unsupervised; make sure you know what is going in; ask them 'what exactly is it that we are buying here?'
"If you are getting it form the chemist make sure not a lot of it goes into the actual mix.
Which? has called for all potentially unsafe children's slimes to be removed from sale.
