My doctor prescribed monodox (doxycycline monohydrate) 75mg== are there any serious side effects?

Serious = quite rare. Anaphylaxis (severe allergic reaction), serum sickness, cardiac sac inflammation (pericarditis), hemolytic anemia, and exfoliative dermatitis (skin feeling off) are serious but quite rare. More often is reflux/gastritis, diarrhea, anorexia, and mild rashes are common but often tolerable. If severe side effects occur, stop med and consult doc asap. Good luck.