01OverviewDefinition, clinical context and the essential points that orientate the chapter.
The diagnostic clue is fixed geography. A round or oval sharply bounded plaque becomes painful, itchy or burning, sometimes develops a central blister, and later leaves brown-grey pigment. With another dose, inflammation reappears in the old mark within hours, although new sites may join. Lips and genital mucosa can be affected and residual pigment may be less obvious there.
Construct an episode-by-episode exposure table rather than inspecting only current prescriptions. Match analgesics, antimicrobials and over-the-counter combinations with recurrence. A consistent history can be diagnostic. Specialist lesional patch testing has limited sensitivity, and oral provocation is reserved for carefully selected low-risk cases because it deliberately reproduces disease.
Clinical decisions in Fixed drug eruption depend on trajectory and consequence. Re-examine evolving skin, repeat focused systemic assessment when the patient changes, and reconcile every result with morphology and timing; a normal early test cannot neutralise worsening pain, mucosal injury or organ dysfunction.
Key points
- A fixed drug eruption recurs at the same sharply demarcated skin or mucosal site after re-exposure and usually leaves residual hyperpigmentation.
- Active lesions may be red, violaceous, dusky brown or blistered; ask the patient whether a dark mark was present before the current flare.
- Search intermittent medicines, analgesics, antibiotics, supplements and combination cold remedies used in the hours or days before every episode.
- One episode may involve one plaque, but additional sites often appear with later exposures and generalised bullous disease can be life-threatening.
- Do not perform casual oral re-challenge when the original reaction was bullous, generalised or involved mucosa.
- Record the specific suspected drug, recurrence pattern, sites, severity and uncertainty, with photographs obtained after consent.
- Fixed drug eruption must be assessed by lesion duration, onset, distribution, symptoms, mucosal findings, systemic physiology and the full medicine timeline rather than by colour alone.
- For Fixed drug eruption, document the working diagnosis, excluded emergencies, uncertain culprit or trigger, treatment response and the exact safety-net given.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Medicine-specific recurrence
Analgesics, antibiotics and many other medicines can activate resident memory T cells at a previously sensitised skin or mucosal site.
Intermittent hidden exposure
Over-the-counter cold remedies, combination analgesics and as-needed medicines often explain repeated episodes whose culprit is absent from the routine prescription list.
Cross-reactive structures
Chemically related medicines may occasionally trigger lesions, but avoidance should be based on documented exposure and specialist assessment rather than uncritical class labelling.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Resident memory response
Drug-specific CD8-positive T cells persist in resolved skin and rapidly reactivate when the culprit is reintroduced.
- 2Local keratinocyte injury
Cytotoxic signalling damages epidermal cells within a sharply demarcated area, causing dusky inflammation, blistering or erosion depending on severity.
- 3Accelerated re-exposure
The first recognised episode may appear after days, while subsequent lesions often recur within hours at exactly the same sites.
- 4Residual melanosis
Inflammation transfers pigment into dermis and alters melanocytes, leaving a sharply outlined brown or grey mark after active tenderness resolves.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
A sharply bounded plaque becomes inflamed at exactly the location of a previous residual mark after each exposure.
Violaceous, grey-brown or dark central colour may blister or erode while the surrounding margin remains clearly demarcated.
A smooth brown or slate macule remains after pain and elevation resolve and can identify old sites between attacks.
Recurrent oral or genital erosions may occur with little cutaneous disease and require consented examination and functional assessment.
Multiple tender dusky plaques with extensive blisters, detachment or mucosal injury require urgent SJS or TEN-level assessment.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Episode-specific medicine tableFirst step - Why
- Demonstrate repeated exposure before recurrence at a fixed site.
- Interpretation and limitations
- Include dose time, over-the-counter brands and ingredients; memory can falsely link the most memorable prescription while missing an intermittent analgesic.
- 02
Consented serial photography - Why
- Confirm anatomical recurrence and evolution to residual pigment.
- Interpretation and limitations
- Use stable localisation and lighting; pigment change can be more visible than acute redness in deeply pigmented skin.
- 03
Skin biopsy of an active lesion - Why
- Support a cytotoxic interface drug reaction when diagnosis remains uncertain.
- Interpretation and limitations
- Histology overlaps erythema multiforme and epidermal necrolysis and cannot independently identify the culprit medicine.
- 04
Specialist lesional patch testing - Why
- Seek support for a selected culprit without systemic exposure.
- Interpretation and limitations
- Testing is placed on a previous lesion site and sensitivity varies by drug; a negative result does not exclude fixed eruption.
- 05
Supervised drug provocation in selected cases - Why
- Clarify an important low-risk uncertainty when specialist benefit outweighs harm.
- Interpretation and limitations
- Avoid provocation after generalised bullous or severe mucosal disease; emergency facilities and a predefined stopping plan are essential.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Arthropod bite and trauma
A solitary inflamed plaque or blister may reflect bite, burn or friction, but exact recurrence after the same medicine exposure supports fixed eruption.
Herpes simplex
Recurrent grouped painful vesicles at a similar mucocutaneous site can mimic fixed eruption and may require viral sampling from a fresh lesion.
Erythema multiforme
Multiple acral typical targets, often after herpes infection, have concentric zones and a broader distribution than a single sharply fixed drug plaque.
SJS and TEN
Widespread tender atypical targets, mucosal loss and epidermal detachment signal a more dangerous necrolytic reaction requiring emergency classification.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Localised stable episodeStop the likely trigger and calm inflammationFirst stepOne or few fixed plaques occur without fever, mucosal loss, widespread blistering or systemic illness.+
- 1Construct a precise recurrent-exposure history, stop the likely non-essential culprit and identify all combination products containing it.
- 2Use cool care, emollient and a site-appropriate topical corticosteroid for short-term symptoms while monitoring for new lesions.
- 3Photograph with consent, explain residual pigment and arrange review if the eruption spreads, blisters or affects mucosa.
02Extensive or bullous episodeTreat as a severe cutaneous reactionMany plaques, significant blistering, detachment, pain or mucosal erosions appear.+
- 1Stop suspected and non-essential medicines immediately, assess ABC, fluid status and exposed body-surface area, and admit urgently.
- 2Obtain dermatology input, biopsy if it will clarify classification and provide skin-failure supportive care while excluding SJS and TEN.
- 3Create a prominent allergy record and prohibit unsupervised re-exposure because subsequent recurrence may be rapid and extensive.
03Future medicine decisionVerify culprit without unsafe challengeThe reaction has resolved but the suspected medicine may be clinically valuable or several exposures remain plausible.+
- 1Refer specialist drug allergy or dermatology with images, dates, exact products and a description of bullous and mucosal involvement.
- 2Consider lesional patch testing or carefully controlled provocation only when severity and expected benefit make that approach acceptable.
- 3Give written avoidance advice that names confirmed ingredients and separates them from unrelated medicine classes.
Key medicines and prescribing safety2 treatments · regimens, roles and cautions+
Betamethasone valerate 0.1% cream
Apply thinly to a localised intact plaque once or twice daily for the shortest effective course, not exceeding four weeks under the licensed product directions.Avoid application to eroded mucosa, infected skin, face or large areas unless directed; review atrophy and systemic absorption, especially under occlusion.
Cetirizine 10 mg tablets
Adults and adolescents aged 12 years and over take 10 mg orally once daily, with renal adjustment as specified by the product information.Review drowsiness, alcohol, driving, renal function, pregnancy and breastfeeding; do not interpret symptom relief as evidence that bullous progression is safe.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Generalised bullous disease
Extensive recurrent lesions can blister and detach across large areas, causing fluid loss, infection risk and overlap in appearance with epidermal necrolysis.
Mucosal scarring
Repeated erosive disease at genital or oral sites can cause pain, dyspareunia, urinary symptoms, adhesions and functional impairment.
Permanent pigment alteration
Each recurrence may deepen post-inflammatory hyperpigmentation, creating a lasting visible footprint after the acute lesion has healed.
Repeat prescribing
Failure to connect a residual mark with intermittent exposure can lead to repeated deliberate re-challenge and increasingly rapid recurrence.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Review local fixed lesions until blistering has stopped and re-epithelialisation occurs, with earlier contact for new sites or mucosal pain.
- Update the allergy record with exact ingredients and common combination brands likely to cause accidental repeat exposure.
- At later visits, ask about recurrence and inspect residual sites because the pigment footprint can verify an otherwise forgotten episode.
- At every review of Fixed drug eruption, record lesion evolution, new mucosal or systemic features, medicine changes, treatment adherence and adverse effects.
- Give a named route for urgent reassessment if breathing, circulation, fever, skin pain, blistering, facial swelling, reduced urine output or other organ symptoms develop during Fixed drug eruption.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
The old mark is evidence
A patient may present only with residual pigment, and careful questioning can reveal repeated inflammation after the same intermittent tablet.
New sites accumulate
Re-exposure commonly reactivates every old focus while recruiting additional lesions, so apparently local disease can become progressively widespread.
Mucosa may not pigment
Lack of a visible residual oral mark does not exclude fixed mucosal disease when recurrence and exposure timing are convincing.
Patch testing uses old skin
When specialists test, application to a resolved lesion site is more informative than ordinary unaffected back skin.
Brands conceal repeated ingredients
Different cold or pain products may contain the same culprit and explain recurrence despite changing package names.
11Common pitfallsFrequent interpretation and management errors.
- 01
Missing over-the-counter analgesics because only repeat prescriptions were reviewed.
- 02
Calling a residual brown plaque active allergy and escalating treatment despite absent tenderness or elevation.
- 03
Rechallenging a patient at home to prove the drug association.
- 04
Treating generalised bullous fixed eruption as a few harmless local spots.
- 05
Adding an entire antibiotic or analgesic class to the allergy record without documenting the actual ingredient and uncertainty.