I am not a medical professional and my advice is of one who has no credentials or qualifications for this.

I’ve helped an undisclosed individual with cleaning vials.

This assumes that the vial fluid has not broken down, this instruction is to remove pathogens, not rancid fluid other toxins floating in the vial.

You need a glass filter like this, with the smallest pore size. https://www.hawachfilters.com/glass-fiber-syringe-filters.html

The reason for this is that most bacterium are over half a micron long. You’re unlikely to find anything smaller surviving in the fluids of your medication vial, viruses being even more rare inside.

Afterwards, you’ll need a new sterile bottle to fill.

Ensure your environment is sterile, or as clean as possible without airflow. ensure all equipment is clean and sterilized too.

1. Use an 18g needle on a syringe of sufficent sizr to draw out of the vial contents that either is 1 month old, or has rubber filaments in it from a damaged cap.
2. Once your syringe is filled with the vial contents, remove the 18g needle & install the filter.
3. Install a 25g needle to the M port of the filter. The reason for this is to ensure the smallest hole possible in the new vials seal.
4. Insert into the new vial.
5. At this point, you will want to take a spare 25g needle (no syringe) and insert it into the vial on the opposite side of your filling syringe. This is for pressure difference.
6. In a blood curdilingly slow speed, slowly push all the medication through the filter into the vial. Take it as slow as possible, this is not a fast process, it will take 10+ minutes.
7. Once as much of the medication is through as possible, you have a generally filtered and clean vial of your medication.

The undisclosed individual was able to use the vial to completion without infection or issue, then resumed using clean vials.