Antibiotics start attacking bacteria soon after the first dose, but the relief you feel lags behind because your body still needs time to clear the infection and recover. For many common infections people notice improvement within two to three days of starting treatment.
If you are not feeling any better after roughly three days, are feeling worse, or develop a high fever, severe pain, breathing difficulty or a spreading rash, you should contact a healthcare professional, as the antibiotic may not be the right one or the illness may need reassessment.
Importantly, feeling better is not a signal to stop. You should normally finish the full course exactly as prescribed so the infection is fully cleared and to reduce the risk of antibiotic resistance. Seek urgent care for signs of a severe reaction such as facial swelling, difficulty breathing or fainting.