How long does it take for a penicillin shot to work and is it normal for the injection site to be sore?

A penicillin shot typically begins to work within a few hours, but the full effects may take a day or more to become apparent, depending on the infection being treated. It is normal for the injection site to be sore, swollen, or even slightly bruised for a few days after the shot. If the pain or swelling persists or worsens, it's advisable to consult a healthcare professional.