Dermatologic Therapeutics and Systemic Drug Reactions
Therapeutics
Know mechanisms, monitoring, and major adverse effects of core drugs: topical corticosteroids (potency classes, tachyphylaxis, atrophy), calcineurin inhibitors, methotrexate (folate antagonist; monitor liver and marrow; folic acid supplementation), azathioprine (TPMT deficiency), cyclosporine (nephrotoxicity, hypertension), dapsone (G6PD deficiency, methaemoglobinaemia), isotretinoin (teratogenicity, mucocutaneous effects, monitoring), antibiotics, and biologics (TNF inhibitors and infection/reactivation risk, IL-17 and IL-23 inhibitors).
Systemic drug reactions
Learn the severe cutaneous adverse reactions: Stevens–Johnson syndrome and toxic epidermal necrolysis (epidermal detachment, Nikolsky positive, mucosal involvement), drug reaction with eosinophilia and systemic symptoms (fever, rash, eosinophilia, organ involvement, HHV-6 reactivation), and acute generalised exanthematous pustulosis (pustules, neutrophilia). Also know fixed drug eruption, drug-induced lupus, and lichenoid drug eruption.
Traps
- Confusing DRESS (eosinophilia, organ involvement, delayed onset) with SJS/TEN (mucosal and epidermal detachment).
- Forgetting TPMT testing before azathioprine.
- Missing G6PD screening before dapsone.
- Isotretinoin: pregnancy prevention programme is mandatory.
Rule of thumb
For any severe drug reaction, ask three questions: how much epidermal detachment, is there mucosal involvement, and are there systemic/eosinophilic features? Those answers separate SJS/TEN from DRESS from AGEP.