Introduction
When evaluating patients with possible drug allergy, one examines
the circumstances surrounding the episode, the results of allergy
testing, and at times, deliberate challenge.
Pain killers and antibiotics are the most common causes
of drug allergy
Allergic reactions have been described to a large number of
medicines. Reactions to pain killers/arthritis tablets (such as
aspirin, ibuprofen, naproxen) and antibiotics are the most common,
but reactions have been described to many other medicines, including
some herbal remedies such as Echinacea, Royal Jelly and chamomile.
Who is more likely to suffer drug reactions?
Allergic reactions to medicines are no more likely to occur
in people with other allergies (such as hay fever, asthma or eczema)
than anyone else in the genera population. While there are reports
of some families who have many people with allergic reactions
to medicine, most drug allergies are not inherited.
Evaluating the person with possible drug allergy
When evaluating patients with possible drug allergy, one examines
the:
o History of the episode,
o Results of allergy testing, and at times,
o Deliberate challenge with the medication.
History of the reaction
The timing and the symptoms experienced may help classify
the event as unlikely, possibly or probably representing an allergic
reaction.
Unlikely: For example, a headache or upset stomach after
a medicine might be a side-effect, but is not an allergic reaction.
Possibly: A rash starting a few days after taking a medicine
might be due to the medicine, or due to another cause, such as
an infection being treated.
Probably: The sudden onset of a generalized rash, difficulty
breathing and stomach upset within an hour or so of taking a new
medicine suggests a severe allergic reaction known as anaphylaxis.
Drug allergy may be immediate or delayed
Not all drug allergy is of rapid onset with hives and difficulty
breathing. Some reactions are still causes by an interaction between
the medicine and the immune system, and may only appear after
the medicine has been taken for several days, and sometimes even
after the medicine has been ceased.
Drug allergy testing
Allergy testing can be done for some but not all medications.
Allergy testing has been shown to be useful for assessing possible
allergic reactions to some antibiotics, anaesthetic drugs and
latex. When considering antibiotic allergy testing, one requires
an intravenous solution for accurate testing, and some medicines
come only in tablet form. For other medicines, such as painkillers
(eg. aspirin or tablet-only antibiotics), there is no accurate
test to confirm the presence of allergy.
Antibiotic allergy testing
Unfortunately, allergy testing with antibiotics is not a hundred
per cent accurate. This can be due to:
o The absence of availability of intravenous forms of some medicines
for testing.
o Many patients allergic to medications are allergic to the breakdown
product of the drug, not the native drug itself. It is often difficult
(or impossible) to obtain commercial extracts for allergy testing.
o The passage of time. Many patients with antibiotic allergy will
grow out of their sensitivity with time, with their skin tests
turning negative after a few months. Testing many years down the
track may be negative, even if the person was allergic in the
first place.
o When rash is the main symptom, it is often difficult to determine
whether the cause was the antibiotic, or the infection that prompted
its use.
Interpreting the results of drug allergy testing
In interpreting the results of drug allergy tests, a positive
result usually indicated that the person is sensitive to that
medication. When results are negative, however, the possible explanations
can be that:
o The patient is not allergic now and never was, or
o That the patient was allergic and has grown out of their sensitivity,
or
o The test result is inaccurate.
Challenge
When it is important to prove or disprove sensitivity, deliberate
challenge is sometimes required to determine the presence of absence
of drug allergy. This is normally performed under medical supervision,
using small doses first.
What to do if you suspect a problem with medicines
o If there is a rash, take photos! Often the rash will be
gone by the time you see your doctor.
o Write down the following information:
o What medicine was taken?
o What symptoms did you get, and when?
o How long did they last?
o What was the time relationship between taking the medicine and
the onset of symptoms? 2 hours, 3 days, a week?
o WHY were you taking the medicine?
o Were you taking any other medicines or complementary / herbal
medicines as well?
