Miley Cyrus is in the hospital ... after a sudden and apparently severe allergic reaction to medicine.
Miley was in Kansas City for a concert Tuesday night on her Bangerz tour. It appears Miley was at the arena -- the Sprint Center -- when she fell ill. We're told she had a bad reaction to antibiotics.
Doctors ordered her on bed rest immediately.
Here's what's interesting ... A TMZ employee happens to be in Kansas City and just texted us an hour ago telling us Miley was out shopping with her mom ... so it seems whatever reaction she had was sudden.
The concert has been cancelled. She also cancelled her concert last Monday in Charlotte due to the flu.