Many times when you go to click on your eat or drink options from your inventory, your guy will remove his weapon or whatever hes holding like hes going to eat but just sit there, and the message "you are already doing something" appears, and if you click eat or drink 2 more times it will then actually work.

Also for me at least it seems as if the eat or drink option does not work properly if the item in question is in the "Vicinity" inventory section.