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.
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).