Generally speaking, yes it would be considered a form of euthanasia by the Church. While the Catholic medical ethics does not require extraordinary means to preserve life, ordinary means of care are still morally obligatory. Withholding ordinary care with the intention of causing death is morally considered passive euthanasia.
In the situation of antibiotics, if taking them causes the patient disproportionate pain and suffering (problems swallowing, bad reaction, etc) compared to the expected benefit(s) then they may be morally withheld even if it hastens death. However, if the actual taking of the antibiotics is not problematic then the patient should be treated with them as they ordinarily would.
