side effect from antibiotic

Question
I have a 15year old toy poodle who weighs 10lbs.My vet recently started him on an antibiotic for enlarged prostate which may be due to his age or maybe an infection.I have given him 5days of this medicine.Its Smz Tmp 480mg.He gets 1/4 of a pill in am and pm.Since starting this antibiotic he becomes sleepy and lethargic,sometimes unsteady gait.I called my vet and told her his symptoms and asked if I could cut back on the dose to just 1/4 pill once a day instead of twice a day because of the side effect of this pill he is having.She told me to just continue to give him the dose she ordered and IF he should stop eating and drinking to call her and she will then cut back the dose. Considering my dogs age and considering it is not certain that he does actually have an infection which is causing the enlarged prostate and could be age related and considing he is having side effects do you think me cutting back to 1/4 pill once a day would be a wise choice instead of making this poor dog go thru the side effects ?thanks

Answer
Please forgive the lateness of this answer but my Internet connection has been down completely for two days.

Well TMZ is a sulfa drug and maybe he is not good with sulfa drugs. I take it myself when I have lung infections and it works well.
I would call the vet and tell her you want to switch him to a different antibiotic if she won't let you lower the dose. Enlarged prostates are usually from a dog not being neutered but they can start from infections also.
Has she done blood work to determine if his white blood cell count is up?
I have never heard of this drug causing those kind of side effects but that doesn't mean it isn't possible- anything is!
I wouldn't cut the dose only because it won't do it's job if you do and if he does have an infection it could get worse and start a resistance to antibitoics completely.

You should only have 4 more days on this so try to hang in there with him but certainly call her and ask if there is another drug that will do the same job that he won't have a reaction to.
If he doesn't have an infection he will need to be neutered to make this go away.
I hope he does ok and please let me know what you find out.