Bananas contain a radioactive isotope of potassium called K-40, but it doesn't lead to radiation poisoning.

Do you know why?

Unit to measure radiation exposure is sievert.

According to research, radiation exposure should not exceed 50,000 millisieverts annually.
Eating a banana exposes us to only 0.1 microsieverts. So, bananas won't cause radiation poisoning unless you eat millions of them at the same time.