There are two types of adverse drug reactions.
DIAGNOSING ADVERSE DRUG
REACTIONS
There
are two types of adverse drug reactions. Type A reactions are common,
predictable, usually dose-dependent and appear as excessive manifestations of
the normal pharmacology/toxicology of the drug; they are seldom fatal. Type B
reactions are uncommon, unpredictable, often independent of dose and usually
represent abnormal manifestations of the drug’s phar-macology/toxicology; they
involve relatively high rates of serious morbidity and mortality.
ADRs
frequently mimic ordinary diseases and, if they are uncommon, may easily be
overlooked. They tend to affect the skin, haematopoietic system and lining of
the gut (situations in which there is rapid cell multiplication) or the liver
or kidneys (where drugs are detoxified and excreted). These special sites are
frequently involved in iatrogenic (doctor-induced), type B illnesses, such as
toxic epidermal necrolysis, aplastic anaemia, pseudomembranous colitis,
drug-induced hepatitis or nephritis.
A
high index of suspicion is needed if ADRs are to be successfully diagnosed. The
clinician always has to think: ‘Could this be drug-induced – is this an ADR’.
The question is important, for withdrawal of the cause of an ADR is usually
essential.
Iatrogenic
ADRs are usually uncommon or rare, and this adds to the difficulty of
diagnosis. Some are avoidable, such as skin rashes in patients with glandular
fever given ampicillin. Some are accidental, such as the non-iatrogenic
disaster of an asthmatic given a beta-adrenergic blocking agent by another
member of the family. It is a truism that the detection of common or uncommon
ADRs requires vigilance. Many of the known serious ADRs have been recognized by
astute clinicians with a high level of awareness, and such awareness is likely
to be just as important, as new methods of pharmacovigilance are developed as
it has been in the past.
Linked
with this problem of diagnosing ADRs is the problem of understanding them. Why
does one patient in 10 000 get some bizarre type B reaction, and the rest of
this population not get it? Clearly, our increasing knowledge of clinical pharmacology,
drug metabolism and genetics will contribute to our understanding of these
things, and these subjects are explored in many of the chapters in this book.
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