What is the typical time frame for the development of an exanthematous/morbiliform rash after starting a medication?

The development of an exanthematous or morbiliform rash typically occurs within a time frame of 2 to 14 days after the initiation of a medication. This timeline reflects the delayed hypersensitivity reaction that is commonly associated with such rashes.

When a patient starts a new medication, the immune system may take some time to recognize the drug as foreign and mount an immune response. This response can result in the manifestation of skin rashes that resemble measles, which typically appear days to weeks after drug exposure rather than immediately.

In contrast, immediate reactions, which can occur within minutes to hours of exposure, are more often associated with other types of hypersensitivity, such as anaphylaxis or urticaria, and not with exanthematous rashes. Similarly, while rashes might appear after longer durations, such as a month, that is not typical for the common morbiliform reactions seen with many medications. Thus, the time frame of 2 to 14 days is crucial for identifying a drug-specific exanthematous reaction.