Doxycycline hydrochloride is a common prescription antibiotic for people that is also often prescribed for dogs. Doxycycline is in the family of tetracycline antibiotics sold under brand names Monodex, Vibra-Tabs and Vibramycin. This is a powerful antibiotic that works by restricting the growth of bacteria--both good and bad. However, some vets prefer to use doxycycline rather than any kind of tetracycline for dogs with kidney problems.
Loss of Appetite
"The Pill Book Guide to Medication for Your Dog and Cat" notes that this is one of the most common side effects, and is due to nausea. Dogs may even refuse treats as well as regular food.
Nausea and Vomiting
In order to reduce nausea, you can feed the dog first or mix the doxycycline with food. Unlike tetracycline, doxycyline can be taken on a full stomach.
This is an unusual side effect and may indicate that the dosage is too high. Dr. Mark Papich recommends that any dog on doxycycline that gets diarrhea be taken to a veterinarian (see Resources).
Doxycycline can harm the calcium on teeth and cause them to turn yellow. For this reason, puppies still with their milk teeth usually don't get doxycycline.
Very Rare Side Effects
"The Pill Book Guide to Medication for Your Dog and Cat" notes that the following symptoms rarely happen: the dog becomes more easily easily sunburned, gets liver damage or has changes to blood cells.