GGC Medicines

Adult Therapeutics Handbook

Antibiotic Allergy and Interactions

Antibiotic Allergy and Interactions


  • Specifically ask the patient as to the nature of 'allergy'. Abdominal pain, nausea, vomiting, diarrhoea or dyspepsia does not constitute allergy.
  • Document both the allergy and the nature of the allergy in the patient's medical notes and on the drug kardex.
  • Do not give penicillin, cephalosporin or other beta-lactam* antibiotic if patient has a history of anaphylaxis, urticaria, severe blistering rash, or rash immediately after penicillin administration. Adults with a history of a minor rash (e.g. non-confluent, non-pruritic rash restricted to a small area of the body), or a rash that occurs >3 days after starting an antibiotic course are unlikely to have an antibiotic allergy and therefore the antibiotic should not be withheld unnecessarily for serious infections.
  • Penicillin allergy is reported by 10% of patients. Anaphylaxis (true penicillin allergy) occurs in <1% of treated patients.
  • Co-trimoxazole: rash occurs in >1 in 100 patients prescribed co-trimoxazole. If a rash occurs, discontinue co-trimoxazole immediately.
  • Be aware of the components of antibiotic co-formulations e.g. co-trimoxazole (trimethoprim and sulfamethoxazole), co-amoxiclav (amoxicillin and clavulanic acid), piperacillin with tazobactam.
  • For further advice on antibiotic allergy please contact the allergy service – see Appendix 6 for contact details.

*Beta-lactam antibiotics include: amoxicillin; ampicillin; benzylpenicillin (Penicillin G); co-amoxiclav (Augmentin®); flucloxacillin; phenoxymethylpenicillin (Penicillin V); piperacillin / tazobactam (Tazocin®), pivmecillinam; temocillin; cefaclor; cefalexin; cefotaxime; ceftazidime; ceftriaxone; cefuroxime; aztreonam; meropenem; imipenem with cilastatin, ertapenem (see BNF for more details).

Important Antibiotic Drug Interactions

This is not a comprehensive list; for further information refer to a pharmacist or Appendix 1 of the BNF.

  • Macrolides (e.g. clarithromycin): numerous interactions (some potentially life-threatening) via;
    • Enzyme inhibition e.g. carbamazepine and simvastatin (see BNF Appendix 1).
    • QT prolongation e.g. citalopram, fluconazole, quinolones; seek advice from pharmacy. Consider other medical risk factors for QT prolongation. Also see Assessing Medicines on Admission in Acute Patients and Medicines Update Extra (MUE 08) Drug induced QT prolongation at
  • Rifampicin: numerous interactions through enzyme induction (see BNF Appendix 1). The interacting effect of rifampicin can persist for many days after stopping rifampicin therapy
  • Statins: Avoid concomitant use with macrolides and sodium fusidate (consult BNF for details).
  • Quinolones (e.g. ciprofloxacin and levofloxacin):
    • QT prolongation (as for macrolides, see above)
    • Lower seizure threshold (see BNF Appendix 1).
    • Absorption reduced by oral iron, calcium, magnesium and some nutritional supplements.
  • Warfarin: INR may be altered by many antibiotics, particularly if a course is prolonged (check BNF Appendix 1).
  • Doxycycline: absorption reduced by oral iron, calcium, magnesium and some nutritional supplements.
  • Oral contraceptive pill: no additional contraceptive precautions are now required when combined oral contraceptives are used with antibiotics which do not induce liver enzymes, unless diarrhoea or vomiting occur (see BNF for advice).

Content last updated December 2018