Consumer watchdog Which? tested 11 toy slimes on sale in the UK and found that eight contained excessive levels of a chemical called boron.
High levels of boron can cause vomiting, irritation, and diarrhoea and, according to the European Union, very high levels can cause infertility and even harm children in the womb.
Toys sold in the EU must contain less than 300mg of boron per kilo.
But the survey found eight of the 11 slimes tested contained up to 1,400mg – four times the allowed limit.
The boron comes from the borax used in slime to give it its distinctive texture.
The high levels were not just limited to the UK, with Italian authorities also finding unsafe levels in their own slime tests.
Which? has urged parents to be wary of boron when buying slime.