Trimethoprim, an antibiotic, is often used in combination with a sulfa drug, such as sulfamethoxazole or sulfadiazine. The combination is used to treat susceptible bacterial and protozoal infections in dogs, cats, and other pets. n dogs, keratoconjunctivitis sicca (KCS, dry eye) may rarely occur with long-term use of trimethoprim/sulfa. Stop giving the medication and consult your veterinarian if your pet experiences discharge from the eye, redness of the eye, squinting, or other signs related to the eye. In long-term therapy, your veterinarian may request periodic exams to monitor tear production. Also contact your veterinarian if your pet experiences pale gums, bleeding tendencies, fever, depression, loss of appetite, vomiting, diarrhea, sore joints, lameness, skin rashes, increased thirst or urination, skin rashes with possible sensitivity to sunlight.