The death of a cat after eating doxycycline may be caused by an overdose or a serious condition. The specific cause of death needs to be determined based on the actual condition of the cat. Doxycycline is generally used to treat colds or upper respiratory tract infections. After giving it to cats, they need to drink more water to avoid burning the esophagus, and the dosage must be controlled, otherwise it will cause vomiting and diarrhea. It is best not to feed doxycycline to kittens. Doxycycline.