Drug Eruptions (Drug Rashes)

What happens in a drug eruption

Most drug rashes are caused by the immune system reacting to a medicine. Some reactions are immediate, driven by IgE antibodies (hives and anaphylaxis). Most are delayed, driven by T cells, appearing days to weeks later. A few are not allergic at all, such as sunburn-like reactions from photosensitizing drugs.

Common types of drug eruptions

Severe cutaneous adverse reactions

These are rare but potentially life-threatening:

• Stevens-Johnson syndrome and toxic epidermal necrolysis: flu-like symptoms, then painful red or dusky skin that blisters and peels, with sores in the mouth, eyes and genitals. Common triggers include allopurinol, carbamazepine, lamotrigine, phenytoin, sulfonamide antibiotics and nevirapine. Treated in hospital, often in a burn unit
• DRESS (drug reaction with eosinophilia and systemic symptoms): a widespread rash, fever, facial swelling, swollen lymph nodes and inflammation of the liver, kidneys or other organs, usually 2 to 8 weeks after starting anti-seizure drugs, allopurinol, some antibiotics or sulfasalazine
• AGEP (acute generalized exanthematous pustulosis): many tiny pustules on red skin with fever, usually within days of antibiotics
• Anaphylaxis: hives with throat swelling, breathing difficulty or low blood pressure

Risk factors

• previous drug reactions
• HIV infection, which greatly increases the risk of sulfonamide rashes
• viral infections at the same time (for example amoxicillin given during mononucleosis often causes a rash that is not a true allergy)
• certain genes (HLA types) that predispose to SJS from specific drugs, such as HLA-B*15:02 with carbamazepine in people of Southeast Asian ancestry
• older age and taking many medicines

Diagnosis

Doctors identify the likely cause by:

• reviewing all medicines, supplements and over-the-counter products taken in the past 8 weeks, with start dates
• matching the rash type and timing to known drug patterns
• checking for warning signs: mucous membrane involvement, blisters, fever, organ involvement
• blood tests for eosinophils, liver and kidney function in suspected DRESS
• a skin biopsy in uncertain or severe cases

Allergy testing, including skin tests and supervised drug challenges, can later confirm or rule out allergy, especially for penicillin. Most people labeled penicillin-allergic are not truly allergic when tested.

Treatment

• Do not stop a prescribed medicine on your own if it is important (for example, an anti-seizure drug); contact your prescriber, who can switch you to a safe alternative
• Mild rashes: stop the suspected drug as advised, use oral antihistamines for itch, moisturizers and medium-strength steroid creams
• Hives and anaphylaxis: antihistamines; epinephrine and emergency care for anaphylaxis
• SJS, TEN and DRESS: immediate stopping of the drug, hospital care, wound and eye care, fluids, and treatments such as steroids or other immune-modulating drugs depending on the condition

Most mild drug rashes fade within 1 to 2 weeks after stopping the medicine, sometimes with peeling as they resolve.

After a drug eruption

• record the drug name, the reaction and the date
• tell every doctor, dentist and pharmacist
• wear a medical alert bracelet after a severe reaction
• never take the drug again after SJS, TEN or DRESS, and ask about related drugs to avoid
• ask about allergy testing if you have a penicillin label, since removing an incorrect label opens up better antibiotic choices

Frequently asked questions

What does a drug rash look like?

The most common drug rash is a widespread, itchy, red rash of small flat and raised spots that join together, often starting on the trunk. Other types include hives, round dark patches that recur in the same place (fixed drug eruption), and sunburn-like rashes on sun-exposed skin.

How long after taking a medicine can a rash appear?

Hives can appear within minutes to hours. The common measles-like drug rash usually appears 4 to 14 days after starting a medicine. DRESS can appear 2 to 8 weeks after starting. A rash can occasionally appear shortly after stopping a medicine.

How long does a drug rash last?

A mild drug rash usually fades within 1 to 2 weeks of stopping the medicine, sometimes with peeling. Severe reactions such as DRESS can take weeks to months to fully settle and need medical follow-up.

When is a drug rash an emergency?

Get emergency help for facial or throat swelling, breathing difficulty, blistering or peeling skin, sores in the mouth, eyes or genitals, or a rash with high fever and feeling very unwell. These can signal anaphylaxis, Stevens-Johnson syndrome or DRESS.

Is a drug rash the same as a drug allergy?

Many drug rashes are allergic reactions, but not all. Some rashes, such as the one many people get when taking amoxicillin during mononucleosis, are not true allergies. Allergy testing can clarify whether you need to avoid a medicine in the future.