Hello and welcome to Ask A Doctor service. My name is Dr Rahman, I have read your question and here is my advice. Being allergic to one drug slightly increases the risk of other drugs intake. However, if you are only allergic to sulfa group of drugs, there is nothing like sulfa in either Polymycin and Trimethoprim so I suppose it would be safe. But remember, Trimethoprim also comes in combination with Sulfamethoxazole which is sulfa group so just check it. If it is not such a combination, I am confident there would be no cross reaction between these. Still, as it cannot be ruled out for any drug: start it in low doses and if you feel any irritation you can stop it otherwise increase the dose to a routine dose. I hope I have answered your question. I will be glad to answer any further question about this. Regards, Dr. Rahman