Chapter: Clinical Dermatology: Drug eruptions
Drug eruptions Course and Differential diagnosis
Course
The
different types of reaction vary so much that a brief summary is not possible.
If a reaction occurs during the first course of treatment, it
characteristic-ally begins late, often about the ninth day, or even after the
drug has been stopped. In such cases, it has taken that lag time to induce an
immune reaction. In previously exposed patients the common morbilli-form
allergic reaction starts 2–3 days after the admin-istration of the drug. The
speed with which a drug eruption clears depends on the type of reaction and the
rapidity with which the drug is eliminated.
Differential diagnosis
The
differential diagnosis ranges over the whole subject of dermatology depending
on which disease is mimicked. For instance, toxic erythema reactions can look
very like measles, pityriasis rosea or even secondary syphilis. The general
rule is never to for-get the possibility of a drug eruption when an aty-pical
rash is seen. Six vital questions should be asked (Table 22.2).
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