Parents should be wary that some children's slime products may contain higher than recommended levels of a chemical, a consumer watchdog has said.
Which? said that parents should be able to buy toys without being fearful that they could cause their children harm.
The consumer body called for "fundamental changes" to the product safety system after it found that some products had higher than recommended levels of boron.
Slime and slime products have become increasingly popular among youngsters, but Which? say that exposure to excessive levels of boron could cause irritation, diarrhoea, vomiting and cramps in the short term.
The EU states that exposure to very high levels of boron may impair fertility and may cause harm to the unborn child in pregnant women.
Compounds of boron can be used in eye drops, mild antiseptics, washing powders.
Following the craze that has led to millions of YouTube views and Instagram hashtags, the consumer champion tested 11 popular slime products for boron. Boron is found in borax – a common ingredient in slime that helps to create its stickiness.
A European Union safety directive sets out how liquid or sticky toys should contain no more than 300mg/kg of boron.
The consumer champion set out to investigate whether some children's slime products contained the recommended safe levels.
It found that eight out of 11 toy slime products tested exceeded limit.
Which? said that Toysmith Jupiter Juice had more than four times the permitted level of boron with 1400mg/kg.
This was followed by CCINEE Pink Fluffy Slime, which was found to contain 1000mg/kg, and Cosoro Dodolu Crystal Slime Magic Clay, which was found to 980mg/kg, Which? added.
It said that all eight products that failed were purchased on Amazon.
One product purchased on the online marketplace, Hulk Green Halloween Slime, met the standard.
The products which did not meet the standard have been removed from Amazon's website.
A spokesman for the company said: "All Marketplace sellers must follow our selling guidelines and those who don't will be subject to action including potential removal of their account. The products in question are no longer available."
Slime from high street retailers, The Works and Smyths, which was tested also met the standard.
Which? said it has passed its findings to the Office for Product Safety and Standards.
It also warned that parents making homemade slime should be wary when considering this option.
Some reports have suggested that youngsters have sustained injuries after trying to replicate slime recipes found online.
Nikki Stopford, director of research and publishing at Which?, said: "If you have school-age kids you're probably very well aware of the latest slime craze sweeping the playgrounds. Kids love it.
"Parents buying slime for their children should have peace of mind that these toys are safe, so they will be shocked to find that the health of their children could be put at risk by these slimes.
"There must be fundamental changes to the product safety system.
"Manufacturers must stop making unsafe products and the Government and retailers simply have to do a far better job of getting anything identified as a risk off the shelves and out of people's homes".
A Department for Business, Energy and Industrial Strategy spokesman said: "The Government's top priority is to keep people safe, which is why goods being sold in the UK must meet some of the strictest safety laws in the world.
"The evidence provided by Which? will be considered by the Office for Product Safety and Standards and [it can] take any appropriate action."