Mild reactions, such as a rash or itching, can happen with antibiotics like penicillins. If you notice these, stop taking the medicine and contact a doctor or pharmacist for advice and an alternative.
A severe allergic reaction (anaphylaxis) is a medical emergency. Signs include swelling of the lips, tongue, face, or throat, difficulty breathing or swallowing, a widespread rash, dizziness, or collapse. Call your local emergency number straight away and use an adrenaline auto-injector if you have one.
Always tell every clinician and pharmacist about any antibiotic allergy so it can be recorded, and carry that information with you. A true allergy may rule out a whole class of antibiotics, so accurate records matter.