Maculopapular drug rash (morbilliform drug eruption)

The words describe the rash. Macules are flat spots and papules are small raised ones, and morbilliform means resembling measles. The rash is symmetrical and spreads over the chest, abdomen, back and limbs, often sparing the palms and soles at first, and it is typically itchy rather than painful.

It is usually a delayed reaction mediated by immune cells rather than an immediate allergy. That distinction matters, because immediate allergic reactions (hives, swelling, wheeze) appear within minutes to hours of a dose and can be dangerous in a different way, whereas this rash tends to appear days after starting a medicine.

The rash usually emerges several days to about two weeks after starting a new medicine, and sometimes a few days after it has been stopped. Antibiotics are the group most often involved, with penicillin-type drugs frequently quoted, along with anticonvulsants, allopurinol, sulphonamides and many others. DermNet notes that antibiotics are responsible for most serious drug eruptions and that their use should be limited.

A virus is a frequent confounder. Some people given an antibiotic while they have a viral illness develop a rash that is really due to the virus, and a child labelled penicillin-allergic after such a rash may not be allergic at all. Allergy assessment can sort this out later; see the section on labelling.

Clinicians look for signs that a rash is moving beyond a simple eruption. The NHS says to contact 111 if a rash follows a new medicine and involves sensitive areas or is painful, and to call 999 for signs of a serious allergic reaction.

The key tool is a careful medicine timeline: every prescribed, over-the-counter and herbal product, with start dates and any stop or dose change. Blood tests may check blood counts, liver and kidney function, particularly if fever or feeling unwell accompanies the rash. A skin biopsy is rarely needed and is used when the diagnosis is uncertain or severe.

DermNet stresses that accurate diagnosis is important, because wrongly blaming a drug can deprive a person of a useful medicine, while missing the true culprit risks a repeat reaction when it is taken later.

The central step is stopping the suspected medicine, which the prescriber or pharmacist should decide when it is essential for something serious. Most mild rashes fade within a couple of weeks after stopping. DermNet notes that short-term topical corticosteroids such as betamethasone cream are safe and may reduce symptoms, together with frequent emollients. Oral antihistamines may ease itching, though they do not treat the cause, and a short course of oral steroids is occasionally used for extensive rashes.

• Keep skin moisturised with a plain emollient several times a day.

• Use lukewarm rather than hot water, and avoid scratching.

• Photograph the rash daily to show changes to a clinician.

• Do not restart the medicine or take a leftover supply.

Ask that the reaction is recorded in the GP record with the medicine name, date and what the rash was like. A precise entry such as "mild delayed rash on amoxicillin during a viral illness" helps future prescribers decide whether the medicine can safely be used again, whereas a vague allergy label may restrict choices unnecessarily. Where the rash was severe, specialists advise strict avoidance, and DermNet suggests carrying a wallet card or registering with a drug allergy service. A referral for allergy testing may be offered when the label has real consequences.