Idiosyncrasy

Reactions due to hereditary enzyme deficiencies. Atopy may be a risk factor (aspirin) and concomitant use of β-blockers (radiocontrast media)

Individual idiosyncrasy to a topical or systemic drug

Mechanisms not yet known

Cumulation

Reactions are dose dependent, based on the total amount of drug ingested: pigmentation due to gold, amiodarone, or minocycline; may be related to infusion rate (vancomycin)

Reactions due to combination of a drug with ultraviolet irradiation (photosensitivity)

Reactions have a toxic pathogenesis but can also be immunologic in nature (see Section 10)

Irritancy/toxicity of a topically applied drug

5-Fluorouracil, imiquimod

Atrophy by topically applied drug

Glucocorticoids