If you go to a doctor and demand a course of antibiotics for a viral infection, they have been trained to refuse. Because antibiotics do not treat viruses.

For that matter, if you actually do have a simple bacterial infection and immediately demand a last-resort antibiotic like vancomycin, doctors have been trained to refuse. Vancomycin may work on you, but using it may create bacterial resistant strains that will put others at risk. Resistance is especially a threat if you don't complete your course of antibiotics.

So doctors will offer you a different antibiotic instead, with less risk of creating a resistant strain. Even if you promise to complete your antibiotics, "you get what you get so don't get upset".

People have agency, but so do doctors. Doctors are not supposed to be dispensaries who simply give patients whatever they ask for. Doctors have the right to refuse to provide a prescription that is not in keeping with the standard of care, and offer a different prescription instead. You have the right to find a different doctor, or not see a doctor at all.