In a suspected drug eruption, build the timeline before you name the drug

The commonest error in a drug eruption is not misreading the rash - it is stopping the wrong drug, usually the one started most recently, which is often the one started to treat the illness the rash accompanies.

Build the chart before deciding. Write every drug the patient has taken in the last eight weeks down one column, including over-the-counter analgesics, ayurvedic and herbal preparations, and anything taken intermittently. Against each, write the start date and the stop date. Then write the date the rash began. A morbilliform eruption typically appears four to fourteen days after first exposure to a new drug, and within one to two days on re-exposure - so a drug started the day before the rash, on first exposure, is usually not the culprit, while one started ten days earlier and already stopped often is.

The output that matters is what the patient leaves with. A named drug and drug class written on a card, in the discharge summary and told to the patient out loud is worth more than any investigation, because the next prescriber will not have your notes.

• List every drug from the last eight weeks with start and stop dates, including over-the-counter and traditional preparations
• Date the rash onset precisely and compare it against each exposure - 4 to 14 days for a first exposure, 1 to 2 days on re-exposure
• Do not assume the most recently started drug is the cause
• Give the patient the culprit drug and its class in writing, and say it aloud as well
• Record the reaction pattern in the discharge summary - a future prescriber needs to know whether this was morbilliform or something more serious

Why it matters

The patient will be prescribed again by someone who has not seen the rash, and the only thing that protects them is a name written down.

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