The reason why burnt toast is bad for you is because the burnt toast contains a chemical called acrylamide which has been linked to cancer.
Burnt toast is considered carcinogenic which means it has the potential to cause cancer.
Not everyone who eats burnt toast will get cancer as a result but eating burnt toast can increase your risk of getting cancer.
So it's not recommended to eat burnt toast.
Acrylamide is a chemical that is also found in starchy foods like bread and potatoes, if they're cooked at high temperatures for a long time.
This also includes baking, barbeque, frying, grilling, toasting, or roasting.
You can also find acrylamide in other foods such as biscuits and coffee.