Using leftover antibiotics is risky. The drug may not match the bacteria causing your current problem, the amount left may be too little to treat anything properly, and the medicine may be out of date and less effective.
Taking the wrong antibiotic or an incomplete amount can mask symptoms, delay correct treatment, and help bacteria become resistant. It can also cause unnecessary side effects or a dangerous reaction if you have an allergy you did not know about.
If you think you need antibiotics, see a clinician to be assessed. Return any leftover antibiotics to a pharmacy for safe disposal rather than keeping them at home.