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I understand your frustration. Tumors can come from a wide variety of different tissue and each one behaves differently based on what it is made of and where it is located. Some tumors can grow large quickly and then lose the blood supply, which can lead to infection. The antibiotics can help with keeping that infection low or under control, but it doesn't make the tumor go away. The only ways of making the tumor go away are surgery, chemotherapy or radiation treatment, depending on the type of tumor it is.
If your Veterinarian gave you the refills to help keep infection down in the tumor, I would follow their instructions or call them to see if they think the medication would still be helpful.
I hope this information has been helpful. Do you have any additional questions?