20.4 Dermatological Prescribing: Anti-Infectives, Antihistamines, Antimalarials, Dapsone, Special Populations & Cutaneous Toxicity of Cancer Therapy
Key Takeaways
- Hydroxychloroquine should not exceed 5 mg/kg of actual body weight per day, with baseline eye examination and annual retinal screening after 5 years of use, or sooner with risk factors.
- Dapsone causes dose-related haemolysis and methaemoglobinaemia in all patients, so G6PD activity must be checked before starting and blood counts monitored closely in the first months.
- Colchicine combined with strong CYP3A4 or P-glycoprotein inhibitors such as clarithromycin can cause fatal colchicine toxicity.
- EGFR inhibitors cause a papulopustular acneiform eruption in most patients, and pre-emptive oral doxycycline with emollients, sunscreen, and mild topical steroids reduces its severity.
- Vemurafenib and other BRAF inhibitors used alone can cause keratoacanthomas and squamous cell carcinomas through paradoxical MAPK activation, which is reduced by combining them with a MEK inhibitor.
Last updated: September 2026
20.4 Dermatological Prescribing: Anti-Infectives, Antihistamines, Antimalarials, Dapsone, Special Populations & Cutaneous Toxicity of Cancer Therapy
Topical corticosteroids, calcineurin inhibitors, retinoids, vehicles, immunosuppressants, biologics, and phototherapy are covered in the other therapeutics sections. This section adds the remaining drug groups and the rules for special populations.
Principles of Topical Prescribing
- Choose the vehicle for the site and the state of the skin (see the vehicle table in the topical therapy section), and prescribe an adequate quantity using fingertip units.
- Preservatives and fragrances in topical products are common contact allergens (for example methylisothiazolinone and some parabens in leave-on products).
- Wet wraps and occlusion increase penetration and effect, and also systemic absorption.
- Children absorb more topical drug relative to body weight, because their surface-area-to-weight ratio is higher.
Topical Anti-Infectives
| Group | Examples | Points |
|---|---|---|
| Antibiotics | Fusidic acid, mupirocin, retapamulin, ozenoxacin | Use short courses because resistance develops quickly (fusidic acid-resistant S. aureus is common). Mupirocin is kept mainly for MRSA decolonisation |
| Antiseptics | Chlorhexidine, povidone-iodine, octenidine, hypochlorite | Preferred for many superficial infections to reduce antibiotic use. Chlorhexidine can cause contact allergy and anaphylaxis |
| Antifungals | Azoles (clotrimazole, ketoconazole), terbinafine, ciclopirox, amorolfine, nystatin | Nystatin treats Candida only, not dermatophytes |
| Antiparasitics | Permethrin, ivermectin cream, benzyl benzoate, dimeticone | See the parasitic infections section |
Systemic Anti-Infectives Used in Dermatology
| Drug | Key Adverse Effects and Interactions |
|---|---|
| Doxycycline, lymecycline | Photosensitivity and photo-onycholysis, oesophagitis (take upright with water); avoid in pregnancy and young children (tooth staining); never combine with isotretinoin (intracranial hypertension) |
| Minocycline | Blue-black pigmentation, dizziness, DRESS, drug-induced lupus, and autoimmune hepatitis; generally no longer preferred |
| Macrolides | QT prolongation; clarithromycin and erythromycin inhibit CYP3A4 (interactions with statins, ciclosporin, colchicine) |
| Co-trimoxazole | SJS/TEN, marrow suppression, hyperkalaemia; dangerous with methotrexate (additive folate antagonism) |
| Clindamycin | Clostridioides difficile colitis |
| Flucloxacillin | Cholestatic hepatitis, more common in older people and longer courses |
| Rifampicin | Strong enzyme inducer (reduces hormonal contraception, anticoagulants, and many other drugs); orange urine and tears |
| Terbinafine | Liver injury, taste loss; can trigger subacute cutaneous lupus and psoriasis flares |
| Itraconazole | Strong CYP3A4 inhibitor; avoid in heart failure; capsules need food and stomach acid for absorption |
| Fluconazole | QT prolongation; CYP2C9 and CYP3A4 inhibition |
| Griseofulvin | Enzyme inducer, photosensitivity; teratogenic; men should avoid fathering a child for several months after treatment |
| Voriconazole | Phototoxicity; long-term use is linked to squamous cell carcinoma and melanoma and to periostitis |
Antihistamines
- First-generation (for example chlorphenamine, hydroxyzine): sedating and anticholinergic. Avoid them in older people (falls, confusion) and do not rely on them for night-time itch in the long term.
- Second-generation (cetirizine, levocetirizine, loratadine, desloratadine, fexofenadine, bilastine, rupatadine): first-line for urticaria and can be increased up to four times the licensed dose under the urticaria guideline (see the urticaria section).
- In pregnancy, loratadine and cetirizine have the most safety data.
Antimalarials
| Drug | Key Points |
|---|---|
| Hydroxychloroquine | Up to 5 mg/kg actual body weight per day. Retinopathy screening: baseline examination, then annually after 5 years (earlier with kidney disease, tamoxifen, or higher doses). Also QT prolongation, hypoglycaemia, blue-grey pigmentation, and occasional psoriasis flares |
| Chloroquine | Higher retinopathy risk; dose up to about 2.3 mg/kg actual body weight per day |
| Quinacrine (mepacrine) | No retinal toxicity; causes yellow skin; can be added to hydroxychloroquine in refractory cutaneous lupus. Availability varies |
Antimalarials are used for cutaneous lupus, dermatomyositis skin disease, lichen planopilaris, sarcoidosis, polymorphic light eruption, and porphyria cutanea tarda (low dose).
Dapsone
- Mechanism: anti-neutrophil effects, including inhibition of myeloperoxidase, plus antibacterial action against M. leprae.
- Uses: dermatitis herpetiformis, linear IgA disease, bullous lupus, erythema elevatum diutinum, urticarial vasculitis, other neutrophilic dermatoses, and leprosy.
- Adverse effects:
- Haemolysis (in everyone, dose-related; severe in G6PD deficiency, so test G6PD before starting).
- Methaemoglobinaemia (grey-blue cyanosis, breathlessness; measured with co-oximetry).
- Agranulocytosis, usually in the first 3 months.
- Dapsone hypersensitivity syndrome (a DRESS-like reaction; linked to HLA-B*13:01 in Asian populations), hepatitis, and peripheral motor neuropathy.
- Monitoring: blood count frequently (for example weekly) in the first month, then less often; liver tests and methaemoglobin levels as indicated.
Colchicine and Thalidomide
- Colchicine inhibits microtubule assembly in neutrophils. It is used for Behçet disease, neutrophilic dermatoses, leucocytoclastic vasculitis, and recurrent aphthae. Diarrhoea is common. Strong CYP3A4 or P-glycoprotein inhibitors (clarithromycin, ciclosporin, some azoles) can cause fatal toxicity, and the dose must be reduced in kidney impairment.
- Thalidomide is used for erythema nodosum leprosum, refractory cutaneous lupus, Behçet disease, and prurigo. It is a severe teratogen (phocomelia) and needs a strict pregnancy prevention programme. It also causes dose-related peripheral neuropathy, sedation, and venous thromboembolism.
Special Populations
| Population | Key Rules |
|---|---|
| Pregnancy | Generally acceptable: emollients, topical steroids (use very potent steroids sparingly), azelaic acid, penicillins, cephalosporins, macrolides, prednisolone, narrowband UVB, ciclosporin when essential, certolizumab pegol. Avoid: oral and topical retinoids, methotrexate, mycophenolate, tetracyclines (from the second trimester), JAK inhibitors, thalidomide, antiandrogens, griseofulvin, podophyllotoxin |
| Breastfeeding | Most topical agents are compatible (avoid applying to the nipple area); check each systemic drug in a lactation database |
| Children | Weight-based dosing; higher topical absorption (risk of adrenal suppression and toxicity from salicylic acid or lidocaine-prilocaine cream, including methaemoglobinaemia in infants); avoid tetracyclines in young children |
| Older adults | Reduced kidney function, polypharmacy and interactions, falls with sedating drugs, fragile skin, and practical limits on applying topical treatments |
| Kidney or liver impairment | Methotrexate is contraindicated in significant kidney impairment; adjust antivirals, gabapentinoids, and colchicine; avoid terbinafine in liver disease |
Cutaneous Toxicity of Cancer Therapy
| Drug Class | Typical Skin Effects | Management |
|---|---|---|
| EGFR inhibitors (cetuximab, panitumumab, erlotinib, gefitinib, afatinib, osimertinib) | Papulopustular (acneiform) eruption in most patients, without comedones, on the face and upper trunk; xerosis and fissures; paronychia and periungual pyogenic granulomas; trichomegaly and curly hair | Pre-emptive oral doxycycline (or minocycline) for about 6 weeks with emollients, sunscreen, and a mild topical steroid; dose changes by severity grade. The rash severity often correlates with tumour response |
| BRAF inhibitors (vemurafenib, dabrafenib, encorafenib) | Keratoacanthomas and squamous cell carcinomas (paradoxical MAPK activation), UVA photosensitivity (vemurafenib), keratosis pilaris-like and verrucal keratoses, hand-foot skin reaction, panniculitis | Combining with a MEK inhibitor greatly reduces the proliferative lesions; excise SCCs; sun protection |
| Multikinase inhibitors (sorafenib, sunitinib, regorafenib) | Hand-foot skin reaction: painful hyperkeratotic plaques on pressure areas of palms and soles | Urea creams, padding, and dose changes |
| Chemotherapy (capecitabine, 5-fluorouracil, liposomal doxorubicin) | Hand-foot syndrome (palmar-plantar erythrodysaesthesia): tender red swelling and peeling | Cooling, emollients, dose changes |
| Other chemotherapy effects | Anagen alopecia; taxane nail changes; bleomycin flagellate pigmentation; cytarabine neutrophilic eccrine hidradenitis; toxic erythema of chemotherapy; radiation recall; anthracycline extravasation (dexrazoxane) | Treat by cause |
| Immune checkpoint inhibitors (anti-PD-1, anti-PD-L1, anti-CTLA-4) | Itch and maculopapular rash (the most common), lichenoid and psoriasiform eruptions, vitiligo-like depigmentation (in melanoma, linked to better outcomes), bullous pemphigoid, and rarely SJS/TEN | Grade by severity. Topical steroids for mild reactions; systemic steroids and pausing treatment for severe reactions; many patients can continue immunotherapy |
| Hedgehog pathway inhibitors | Muscle spasms, alopecia, taste change | See the basal cell carcinoma section |
Test Your Knowledge
A 50-year-old woman has taken hydroxychloroquine 400 mg daily for cutaneous lupus for 6 years. She weighs 70 kg and has normal kidney function. What does current practice advise?
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Test Your Knowledge
Two weeks after starting dapsone 100 mg daily for dermatitis herpetiformis, a patient becomes breathless with grey-blue lips. Pulse oximetry reads 88%, but arterial oxygen tension is normal. G6PD activity was normal. What is the most likely cause?
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Test Your Knowledge
A patient taking colchicine 0.5 mg twice daily for Behçet disease is prescribed clarithromycin for a chest infection. Why is this combination dangerous?
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Test Your Knowledge
A patient starting cetuximab for metastatic colorectal cancer asks how to reduce the facial acne-like rash he has been warned about. What is the recommended approach?
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