Sulfamethoxazole/Trimethoprim - can I take this for sore throat, possible strep?

Here's why you should not use it if its what you have leftover. Its not the drug of choice for strep A, you don't have enough to do a full course and that is how you create resistant bacteria. You don't even know if it is strep throat, taking an antibiotic when its not needed also creates resistant bacteria. See a dr. You should not even have leftover antibiotics you're supposed to take them all when they are prescribed.