Registered Users
Healthcare Professionals
Free Registration
Drug Allergies
What is an allergy?
An allergic reaction to medications (drug allergy) is but one of the many types of adverse drug reactions (ADR). ADR refers to all harmful and unintended effects of the drug.
These reactions can be broadly classified into:
- Predictable reactions - More common (accounts for 70-80% of all ADR), dependent on the dose, related to the pharmacological action of the drug and occur in otherwise normal patients.
- Examples of predictable ADR include diarrhoea with erythromycin (an antibiotic), stomach ulcers with non-steroidal anti-inflammatory drugs (NSAIDs) and drowsiness with chlorpheniramine (first generation anti-histamine). These examples are not considered as allergic reactions.
- Unpredictable reactions - Less common (accounts for 20-30% of all ADR), dose independent, unrelated to the pharmacological action of the drug, and tend to occur in susceptible patients. Drug allergy is but one of the types of unpredictable ADR.
Drug allergies are brought about by the immune system producing an antibody called Immunoglobulin E (IgE) in reaction to a particular drug taken, or other cells in the immune system called T cells.
ADRs occur in approximately 10 to 20 per cent of all hospitalised patients and result in 3 to 6 per cent of all hospital admissions.
In
Drug allergies have been estimated to account for 5 to 10 per cent of all ADR. Though drug allergy constitutes a small proportion of ADR, serious allergic reactions can result in death.
Common manifestations
Skin rashes
The most common symptoms or signs of a drug allergy are skin rashes.
Different type of rashes may occur. These include:
- generalized redness (erythema)
- maculopapular rashes
- itchy hives (wheals)
- swelling around the eyes, lips and face (angioedema).
However, all other organs of the body can be affected. Examples of which include fever and abnormal liver function tests.
In rare cases, potentially life threatening skin reactions can occur where there is redness of the eyes, mouth and lip ulcers and erosions, erosions of the skin on the genitals (Stevens Johnson syndrome) or where the skin blisters as with a burns’ patient (toxic epidermal necrolysis).
Reactions to painkillers
Of importance, other non-allergic reactions can have similar manifestations to allergic reactions. One common example is swelling of the eyes after taking painkillers such as Ponstan or Aspirin.
This is not classified as a drug allergy as the immune system does not participate in producing the adverse reaction. Instead, this is known as ‘NSAID intolerance’, an unpredictable biochemical reaction to the class of drugs called NSAIDs painkiller.
Other NSAIDs such as Ponstan, Voltaren and Oruvail, should be avoided as well because they may potentially cause a similar reaction. Even NSAIDs in the form of gels and plasters may cause a similar reaction in extremely sensitive patients.
Alternative analgesics that could be used include Paracetamol or Opioid (such as codeine phosphate and tramadol). However, Paracetamol may have a dose-dependent weak NSAID effect and some patients with NSAID sensitivity may develop a similar reaction.
Recent studies suggest that selective COX-2 inhibitors (Celebrex and Arcoxia) are possible alternatives. However, some patients who are extremely sensitive may develop a similar adverse reaction and therefore should not be used unless proper evaluation has been performed.
Other causes of non-allergic rash
Examples of other causes, which could mimic a rash, are certain infections (viral infections for example). Therefore, it is important to have a proper evaluation performed as unnecessarily labelling a patient with having a drug allergy could result in a limitation of medications that could be used when patients are unwell.
Drugs commonly associated with allergic reactions
Theoretically, all drugs can cause allergic reactions. However, the following medications account for the majority of ADRs:
- antibiotics (Penicillin, Cephalosporin, and Cotrimoxazole),
- anti-seizure drugs such as Phenytoin and Carbamazepine
- Allopurinol (medication used in the treatment of gout).
Diagnosis
Things patients should look out for
The key to the diagnosis of a drug allergy is the patient’s account of the reaction in relation to the intake of the drug.
Drug allergies often occur only five to seven days after commencing the drug unless the patient has received the drug unknowingly in the past, or the patient has previously taken a potentially cross-reacting drug (Penicillin or Cephalosporin).
For certain drugs (anti-seizure drugs or Allopurinol), the onset may occur up to two to three weeks after starting the drug.
It is most important to remember the names of the medications that were given so that the doctor can accurately come to a diagnosis. Depending on the colour of the capsule, tablet, or syrup is not enough, as many medications look alike.
Some of the medication names even sound alike such as Glucophage (brand name for Metformin, a medication for diabetes) and Glucobay (brand name for Acarbose, another medication for diabetes).
When in doubt, bring along all the medications to the clinic so that the doctor can review them. It is also important to inform the doctor if there is a history of ingestion of traditional medications (Jamu for instance) as these have also been reported to cause allergic reactions.
Diagnostic Tests
The gold standard for confirmation of a particular drug allergy is the drug provocation test where patients are given incremental doses of the drug under close medical monitoring. Drug provocation tests are usually used to disprove drug allergies to the less likely culprit drug in patients who have been labelled with ‘multiple drug allergies’.
It may also be used to choose alternative drugs for patients who have developed a reaction to another potentially cross-reacting drug. It should not be used to prove that a patient has a drug allergy where the culprit drug can be confidently identified from the history alone as then patients are subjected to unnecessary risks.
Skin tests are only useful for certain drugs such as Penicillin. Only a few blood tests for drug-specific IgE are available.
Treatment
The most important treatment of a drug allergy is to stop the culprit drug immediately!
For mild reactions, oral anti-histamines and immediate cessation of the suspected drug is all that is usually needed.
For moderate or severe reactions such as anaphylaxis, Stevens Johnson syndrome and toxic epidermal necrolysis, or reactions resulting in abnormal blood counts or tests of liver function, immediate hospitalisation is needed.
Further treatment may include administration of Corticosteroids or Immunoglobulin (for toxic epidermal necrolysis).
Appropriate skin care and hydration may also be needed to prevent secondary bacterial infection in situations where the skin is severely blistered or eroded.
Prevention
Drug Alert Identification
To prevent yourself from accidentally being prescribed the same drug again, ensure that the name of the drug is recorded properly. This drug allergy information has to be shown to the prescribing doctor and pharmacist at every occasion.
The Singapore Medical Association administered the Medik Awas (Drug Alert) identity card system and is the best way of ensuring this, as the card will state the name(s) of the medication(s) the patient should avoid.
To obtain this card, an application form must be filled in by a doctor and submitted by the patient along with a $15 registration fee and a passport-sized photograph. Always present this card to any doctor or pharmacist before you are given a medication.
This is important even if the original reaction was a mild rash as repeat exposure to the drug may trigger a severe systemic reaction.
Desensitisation
In situations where the patient requires a drug that he or she is allergic to, patients can undergo desensitisation provided the original reaction was not a life-threatening one.
Desensitisation is a process whereby a patient who is allergic to a certain drug is given that same drug in small incremental doses either over a few hours (in hospital) or over a few days or weeks (as an outpatient) to make the patient tolerant to the medications.
However, this procedure is only used where there are no alternative medications to the drug such as Aspirin for patients who have recently inserted a drug eluting stent for blocked heart vessels, or Allopurinol for patients with chronic tophaceous gout.
In conclusion, allergic reactions to medications are not uncommon and could be potentially life threatening. All patients should maintain a good record of their allergies and inform their doctor during every consultation to prevent unnecessary adverse reactions.
| We comply with the HONcode standard for trustworthy health information: verify here. |
