About 15 per cent of the UK population report adverse reactions
to medication, but only five per cent are truly allergic reactions.
Adverse reactions usually involve the immune system. Of these
less than one per cent are fatal.
Reactions can be divided into three groups:
Those not related to the drug but coincidental and due to
factors other than the drug. For example, rashes or nausea
associated with the disease and not the medication.
Toxic effects: from taking an overdose; those due to known
side effects; and those due to interactions with other medication
taken at the same time.
Then there are allergic reactions and these adverse reactions
usually involve the immune system.
What are the causes?
These may be due to direct anaphylactic immune reactions such as seen
with penicillin, vaccines, blood transfusions, insulin and intravenous
Triggers include aspirin and anti-inflammatory drugs
Other medicines can trigger histamine release in the body by
non-immune mechanisms and for which no diagnostic blood tests
are available. Triggers include aspirin and anti-inflammatory drugs,
morphine and the opiates, local anaesthetics and some fluids given
intravenously during x-ray.
Drugs that cause allergic reactions:
antibiotics - penicillin, sulphonamides, chloramphenicol and
heart drugs - ACE inhibitors, quinidine, amiodarone, methyldopa
anaesthetic drugs - muscle relaxants, thiopentone, halothane
morphine derivatives - morphine, pethidine and codeine
aspirin-like drugs - diclofenac, ibuprofen, indomethacin
cancer chemotherapy drugs - cisplatin, cyclophosphamide,
antiseptics - chlorhexidine, iodine
vaccines such as tetanus toxoid and diphtheria vaccine
preservatives such as Parabens and Benzakonium chloride
anticonvulsants, antituberculosis medication, streptokinase,
insulin, enzymes and latex
What are the symptoms?
The reaction may progress to life threatening anaphylaxis.
Most reactions occur within one hour and involve measles-like
itchy rashes or urticaria. A severe form may even blister
(Stevens-Johnson Syndrome). The reaction may progress to life
threatening anaphylaxis. Delayed reactions can develop days after
exposure to the drug with generalised dermatitis and damage to
organs such as the kidneys, liver, lungs and blood cells.
What is the treatment?
Treatment involves immediate withdrawal of the implicated drug,
followed by antihistamine medication. In cases of anaphylaxis,
the prompt use of adrenaline and steroids is life saving.
A blood test will confirm if an allergic reaction has taken place.
Unfortunately only penicillin, amoxycillin, sulphonamide and
cephalosporin allergy can be reliably confirmed by RAST testing.
If any other drug is suspected then skin testing, followed by challenge
tests in a hospital will be needed. Patch tests on the skin can test
allergy to certain drugs such as neomycin and the paraben preservatives.
What can be done to prevent drug allergies?
If you are allergic to a member of family of drugs such as penicillin or
aspirin, then all other members of that family should be avoided unless
negative skin or challenge tests have been performed. And if you're
allergic to penicillin, use the Erythromycin family of antibiotics instead.
The same goes for anti-inflammatory medication such as ibuprofen -
only use paracetamol.
It is possible to have an allergic reaction to almost any drug - including
paracetamol - so only use medication if absolutely necessary or if it has
been specifically prescribed to you. Never use someone else's medication
unless you have taken medical advice and are sure it is safe to use.