The best write up on this subject is found in an article published in 2018 in the Clinical Journal of Oncology Nursing (thanks to my friend who co-authored this J. Thompson). The article is entitled: 5-Fluorouracil and Capecitabine: Assessment and Treatment of Uncommon Early On-Set Severe Toxicities Associated with Administration.
If you experience any of the early on-set symptoms, I implore you to report them immediately to your oncologist. Do not feel bashful about reporting into the oncologist -- you should already have a good rapport and if you do not, consider a different physician. Furthermore, do NOT allow the staff or oncologist to easily dismiss your symptoms: time is of the essence in treating potential severe toxicity!
The antidote for any "over-dose" of 5-FU and Capecitabine is Vistogard and this must be administered within 96 hours of your last chemotherapy treatment.