Some children's slime toys contain potentially unsafe levels of a chemical which can cause vomiting and impair fertility, according to Which?.
An investigation by the consumer group into 11 popular slime products found eight contained higher than recommended levels of a chemical called boron.
One product had more than four times the EU safety limit, the tests found.
Which? is advising parents to "approach slime with caution" and said retailers and the government must do more.
It also urged parents to be careful when choosing to make "homemade slime", adding that some ingredients used to make slime - such as some contact lens solutions - contain borax, which is made up of boron.
Slime became one of 2017's biggest crazes, with millions of people sharing pictures on Instagram and watching DIY slime-making videos on YouTube.Which? tested some of the popular slime products on sale for the chemical boron, an ingredient which is often used to make the products sticky.
Excessive levels of boron can cause irritation, diarrhoea, vomiting and cramps, it said.
High levels may also impair fertility or cause harm to an unborn child in pregnancy, according to the European Commission.