Skin is the most frequent target of drug reactions that are reported, may be 
because they are easily detected. Most (probably more than 90%) are related to 
drug hypersensitivity, i.e. an individually tailored, unexpected effect mediated 
by a drug specific activation of the immune response. The clinical presentation 
of "drug eruptions" is highly variable, from the most common transient and 
benign erythema that occurs 6-9 days after the introduction of a new drug in 1 
to 3 % of users to the most severe forms, that fortunately affect less than 
1/10,000 users. Even though there are some overlapping or unclassifiable cases, 
it is important for clinicians to recognize and categorize severe cutaneous 
adverse reactions/SCAR (bullous fixed drug eruptions/bFDE, acute generalized 
exanthematous pustulosis/AGEP, drug reaction with eosinophilia and systemic 
symptoms/DRESS, Stevens-Johnson syndrome/SJS, toxic epidermal necrolysis/TEN). 
First they must suspect rapidly that an unusual eruption with high fever and 
severe constitutional symptoms is caused by a medication and not by an 
infection. Second they have to look for involvement of organs that differ 
according to the type of reaction. Third they can determine a prognosis, the 
mortality rate being virtually 0 for bFDE, 5% for AGEP, 10% for 
"hypersensitivity syndrome"/DRESS and 25% for SJS or TEN. In addition if some 
medications are "usual suspects" for all types (e.g. anticonvulsants), some 
other are more specific of a given pattern (pristinamycine, hydroxychloroquine, 
diltiazem for AGEP, minocycline for DRESS, anti-infectious sulfonamides, 
allopurinol for epidermal necrolysis). The "phenotypic" diversity of the final 
expression drug reactions can be explained by the engagement of a variety of 
cytokines and inflammatory cells and by regulatory mechanisms. For example, 
memory cytotoxic T-Cells are key effectors in both localized blisters of bFDE 
and in extensive blisters of epidermal necrolysis.