Serious or common side effect of doxycycline?

Allergic reactions. Can be very serious side effects if there is: difficulty in breathing, chest tightness, swelling of tongue, diarrhea with or without bloody stools, severe headaches, severe skin reactions like urticaria, blistered or pealing skin. Common and relatively mild se are nausea, vomiting, loss of appetite mild temporary skin rash with or without itching, sensitivity to sunlight. But, overall quite safe.