Allergic drug reactions can be classified based on chronology, mechanism, and clinical phenotypes. The chronology of drug allergic reactions is generally simplified into either immediate or delayed reactions.

Immediate Reactions:

- Occur within 1 hour but in some cases ≤ 6 hours of exposure to the drug.

- Phenotypic reactions:

Urticaria
Angioedema
Bronchospasm
Anaphylaxis

- Often Ig-E mediated

Delayed Reactions:

- Evolve over days, or in some cases, weeks following exposure to the drug.

- Most common clinical phenotype is a benign (e.g., morbilliform drug eruption) exanthem.

- More severe delayed reactions include:

Severe Cutaneous Adverse Reactions (SCARs)
Stevens-Johnson Syndrome (STS)
Toxic epidermal necrolysis (TEN)
Drug reaction with eosinophilia and systemic symptoms (DRESS)
Acute generalized exanthematous pustulosis (AGEP)

- Other more severe delayed reactions:

Serum sickness-like reactions (SSLRs)

SSLRs are characterized by urticaria-like (lesions persist > 24 hours) and erythema multiforme-like lesions, joint inflammation and fever, but unlike serum sickness, nephrotoxicity, and hypocomplementemia are rare.

The immunologic mechanisms for these delayed reactions are likely related to drug-specific T cells including Th1, Th2, and cytotoxic T cells.

- Organ-specific delayed drug reaction phenotypes (often without cutaneous manifestations)

Cytopenias - neutropenia/leukopenia/anemia
Liver injury
Interstitial nephritis
Vasculitis

The chronology of various drug reactions is shown below.