All antibiotics can cause side effects, even at usual doses prescribed by your healthcare professional. These are most likely to include diarrhoea, nausea and/or skin rash, and can occur within 24 hours of starting an antibiotic. If you do have side effects then ask your pharmacist about the best action to take. Some people can have allergic reactions to antibiotics, and these should always be reported to the doctor. If you are prescribed a course of antibiotics it is important to take the whole course as prescribed, unless you are advised to stop by a pharmacist, doctor or nurse.