system
A chat on subreddit /r/NoStupidQuestions
human
They are definitely possible to flushed, but they don't break apart easily in water like things which are supposed to be flushed do. (Namely, toilet paper and poo.)

So, they can help form blockage in your plumbing, and can also gunk up water treatment systems later on. 

Basically, only things that break apart easily in water should be flushed, regardless off how easily other things may go down.
gpt
What kind of tissue are you using that does not break apart easy?