This is a combination of 2 drugs. Sulfamethoxazole and Trimethoprim (SMZ/TMP) It's used to treat respiratory, urinary tract, skin, or gastrointestinal infections, and other conditions. Very very popular drug for humans...at least in those that are not allergic to sulfa.
Make sure Buster has plenty of water to drink.
Other names you may be familiar with are Bactrim and Septra.
If given correctly...12 hours apart (usually it's given twice daily) you really shouldn't have any side effects. A lot of humans are allergic to Sulfa...and so, I'm guessing pets can be too. Call your vet if Buster experiences difficulty breathing, closing of the throat, swelling of the lips, tongue or face, or hives, unusual bleeding or bruising, or yellow skin or eyes. Other side effects are fatigue, dizziness, nausea, vomiting, decreased appetite, diarrhea, weakness, or increased sensitivity to the sun.
What kind of side effects is Buster having?
