What Is A Drug Allergy?
As with other forms of allergy, a drug allergy happens when the immune system reacts to a medication in an unexpected way, leading to symptoms.
Symptoms Of Drug Allergy
Symptoms of allergic reaction to medications may include:
- swelling or
- may affect breathing or cause lowering of blood pressure, in severe cases.
Severe reactions such as this are termed ‘anaphylaxis’ by allergists.
Non-allergic adverse reactions or side-effects are different: these are generally not caused by an immune reaction and in many cases are well known to doctors who regularly prescribe these medications.
How Quickly Do Allergic Reactions To Medication Occur?
Signs of medication allergy occur fairly quickly after exposure, i.e. within an hour. Recovery also occurs fairly rapidly, i.e. over a process of a few hours but more quickly with treatment. Less commonly, drug allergic reactions may be delayed in onset and may persist for days or even weeks.
What Drugs Cause the Most Allergies?
Although almost all medications have the potential to cause allergic reactions the commonest causes are antibiotics, particularly of the penicillin family. Other common causes include non-steroidal anti-inflammatory drugs, e.g. Aspirin and Ibuprofen.
Although local anaesthetic drugs are a common cause of referral to allergy clinics, most such adverse reactions are not due to an allergy.
General anaesthetic allergic reactions, whilst not commonplace are often complex since reactions generally occur after administration of multiple different medications, often simultaneously. These reactions can be severe or anaphylactic and pinpointing the exact cause is extremely important before any further anaesthesia.
Occasionally, some medications may cause rashes that closely mimic those seen in true allergic reactions such as itching, redness and rashes without being true allergic reactions. Examples of this include intravenous dyes (known as ‘contrast’) used during scans and drugs belonging to the opioid family.
Many misconceptions surround drug allergy, including amongst healthcare professionals. For example, a common misconception is that reactions to intravenous contrast are linked to shellfish allergy, although this not the case.
Differences Between Medication Side Effects And An Allergic Reaction to Medication
Distinguishing between medication side effects and an allergic reaction to medication often requires a clinical assessment by an allergist. Typically, this will involve taking a detailed history which will include the speed of onset and the symptoms which occurred. True allergic reactions tend to manifest with predictable symptoms or physical signs.
However, further testing may be required to either rule in or rule out a true allergy. This may involve skin prick testing and in some cases an injection of a tiny quantity of the medication under the skin, known as an intradermal test. If these are negative, testing may progress to a controlled exposure to the medication under medical supervision, a procedure known as a ‘challenge’.
Drug challenges such as this generally start with administration of a very small dose of the suspect drug, which is then gradually increased until the full dose is taken. Patients are then monitored for at least one hour, sometimes longer afterwards.
How Are Drug Allergies Treated?
The most important aspect of managing drug allergies is firstly achieving an accurate diagnosis. Many patients are labelled as being allergic to penicillin, for example, but in most cases, this is on the basis of a childhood rash and subsequent investigation will often rule out a true allergy. Commonly, the original event (usually a rash), likely occurred due to an infection rather than a medication used to treat that infection.
With local anaesthetics most patients will turn out not to be allergic on testing, which can be a huge relief for those about to undergo procedures including dental work.
If a true drug allergy is confirmed on testing, the next step is to identify safe alternative drugs which can be used instead. In specific but rare situations, no alternative may exist, particularly with patients who are severely unwell in hospital. In those circumstances, patients may be desensitised to a particular drug for the brief period of treatment.
However, drug desensitisation is not usually performed outside this specific situation. An exception can be patients who require Aspirin at a low dose but as a long-term treatment, e.g. because of underlying cardiovascular problems. In that situation, patients are sometimes administered increasing amounts of Aspirin in hospital and then continue on a daily dose at home.
Professor Adam Fox
Date reviewed: December 2021
Adam Fox is a Professor of Paediatric Allergy with over 20 years experience in both the NHS and private sector. Professor Fox is Commercial Medical Director at Guy’s & St Thomas’ Hospitals NHS Foundation Trust and Professor of Paediatric Allergy at King’s College London and the founding Director of the KCL Allergy Academy, a postgraduate educational programme, which was a finalist at the BMJ Awards in 2018.
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