I thought this was curious since fennel is something I occasionally use in cooking, so I "Binged" it. For most people, eaten in normal amounts, there's nothing known to be unsafe about fennel. (Some people are allergic to it though apparently.) In fact, eaten in normal amounts there are a number of health benefits. On the other hand, eaten in large quantities, it can produce some side effects as I understand it. For one thing, it is apparently high in estrogenic properties and can, as I understand it, increase women's estrogen levels. It can also interact with some medications... again... if consumed in large quantities. Here's a link to a Healthline article on the subject:

10 Science-Based Benefits of Fennel and Fennel Seeds