Different types of reaction
A medicine can cause adverse effects through different mechanisms. Some are related to its usual action; others involve an immune reaction and are allergic reactions.
A rash, itching, swelling or difficulty breathing after taking a medicine warrants assessment. A reaction can appear quickly or later. Its timing, other medicines taken and the circumstances help guide investigations.
If you have a new reaction, seek medical advice promptly about what to do. Never take a suspected medicine again on your own to check whether you are allergic to it. A healthcare professional will tell you what to do about your current treatments.
Your history is central to the assessment
The doctor will go through the timeline with you: the medicine taken, the reason for treatment, the dates of doses, when symptoms appeared and how they developed. Reactions from the past can also be discussed.
To prepare for this conversation, gather the following if possible:
- The names of the medicines involved and a list of all treatments taken at the same time, including those bought without a prescription.
- The dates and symptoms, even if approximate for an older reaction.
- Reports from consultations, emergency care or hospital stays, and any available test results.
- Photographs of a rash or swelling, if you have them.
You do not need all of these documents to book a consultation. The available information helps determine whether further assessment would be useful and under what conditions.
Tests chosen on a case-by-case basis
Depending on the medicine, the type of reaction and its severity, skin tests, laboratory tests or a drug challenge may be suggested. They are not always necessary and are not suitable in every situation.
A drug challenge involves giving the medicine in a medical setting with appropriate monitoring. Its suitability and safety are assessed beforehand. It must not be attempted at home.
If specialised tests are needed, the practice will explain where they can be carried out and how to prepare.
Clarifying future precautions
The assessment may help identify medicines to avoid and allow alternatives to be discussed. It may also show that a reaction was not an allergy. Some situations remain uncertain, and this will be explained to you.
The conclusions and relevant instructions are shared with you so that you can pass them on to the professionals caring for you. If you have been given an allergy document, keep it and show it at medical appointments and at the pharmacy.
For medicines to take or stop before a possible test, follow the practice’s instructions. Do not make changes based only on the general information on this website.
General information, not a substitute for a medical consultation. Editorial update: .
Consultations are provided by Dr Camille Beniada, FMH specialist in allergology and clinical immunology. Read about her background and her scientific publications.
Further reading: aha! — Medication allergies and intolerances (in French) · HUG — Allergology unit (in French) · aha! — Anaphylaxis (in French)