a lot of antiseptics found in mouthwash, while being excellent at killing oral germs, will also kill normal, neccessary bacteria in your stomach, as well as being irritating to your gastrointestinal tract and maybe upsetting your tummy
plus another bunch of bullshit like highish levels of preservatives/excipients that arent great to swallow a mouthful of (the little bit on your tongue after you spit doesnt count much)
byeeeeeeeee