Dear Customer , This is Dr. S. Tomar from healthcaremagic.com s medical team and I ll be your medical advisor for this question. First of all thank you for sharing your concern with us and showing trust on us. Anaphylaxis is very rare side effect associated with acyclovir . Its so rare that I would recommend you that you can take this medicine without worrying . If you have any symptom like severe itching , breathing difficulty , swelling on face/ tongue / throat , dizziness after taking acyclovir then consult your local emergency services. I would again inform you that such reactions are very very uncommon with this medicine so you can start taking these medicine as prescribed by your GP . Your GP has prescribed acyclovir because the benefits are greater than risk of side effects . Your allergic history is also not a contraindication for use of acyclovir , so you can start it . This is my initial response and discussion is open until you have any query or need further clarification. I wish you good health . Thank you for your question and feel free to contact further for any question . sincerely, Dr. Shardendu Tomar M.D.