Fixed Drug Eruption

What happens in a fixed drug eruption

FDE is a delayed immune reaction. Memory T-cells remain in the skin at the site of a previous reaction. When the same drug, or sometimes a chemically similar one, is taken again, these resident cells reactivate within 30 minutes to 8 hours (usually within a day), damaging skin cells in exactly the same spots. New patches can also appear with each exposure.

Symptoms

• one or a few well-defined round or oval patches, 1 to 10 cm across
• red, then violet or dusky brown color
• burning or itching before the patch appears
• swelling, and sometimes a central blister that breaks to leave a raw area
• most common on the lips, genitals (especially the penis), hands, feet and face, but can appear anywhere
• heals over 1 to 2 weeks, leaving gray-brown or slate discoloration (post-inflammatory hyperpigmentation)
• the mark darkens with each recurrence

People with darker skin may notice the persistent dark mark more than the initial redness.

Generalized bullous fixed drug eruption

A rare, severe form with many large blistering patches over the body. It can resemble Stevens-Johnson syndrome and needs hospital assessment, especially in older people.

Common trigger medicines

Over-the-counter painkillers and occasional antibiotics are easily overlooked as triggers because they are taken intermittently.

Conditions that look similar

Diagnosis

Diagnosis is usually clinical, based on the appearance and the history of recurrence in the same place after a medicine. Your doctor will ask about every medicine, supplement, over-the-counter painkiller and food taken in the days before each flare.

Further steps:

• skin biopsy if the diagnosis is uncertain
• patch testing on the previously affected skin can sometimes identify the culprit
• oral provocation (rechallenge) with a small dose, under specialist supervision only, and not after severe reactions

Treatment

• Stop the culprit drug and avoid it, and chemically related drugs, in future
• Topical corticosteroids for itching and inflammation
• Wound care for blistered or eroded areas: gentle cleansing and nonstick dressings
• Oral antihistamines for itch
• Systemic steroids or other treatments in hospital for generalized bullous FDE
• Record the drug allergy in your medical records and tell pharmacists

The dark mark fades slowly over months. Sun protection helps it fade faster.

Outlook

FDE is usually harmless and does not recur if the trigger is avoided. Repeated exposure leads to more patches and darker marks. Generalized bullous FDE carries more risk and needs specialist care.

Frequently asked questions

What is a fixed drug eruption?

It is a skin reaction to a medicine that causes round, dusky red to purple patches, which reappear in exactly the same spots each time the medicine is taken. It is a delayed immune reaction involving memory T-cells in the skin.

What medicines commonly cause fixed drug eruption?

Common triggers include trimethoprim-sulfamethoxazole and other antibiotics, NSAIDs such as ibuprofen and naproxen, acetaminophen, tetracyclines, barbiturates and some anticonvulsants. Rarely, foods or food dyes cause it.

How long does a fixed drug eruption last?

The active red or blistered patch usually heals within 1 to 2 weeks after stopping the drug. The gray-brown discoloration it leaves can last months or longer and may darken with each new episode.

Can a fixed drug eruption be on the genitals or lips?

Yes. The genitals, especially the penis, and the lips are among the most common sites. It can be mistaken for herpes, but it recurs only after taking the trigger medicine.

How is a fixed drug eruption treated?

The key is identifying and stopping the drug. Steroid creams reduce inflammation and itch, and blistered areas need gentle wound care. Widespread blistering forms need urgent medical care.