What are some clues to help distinguish viral from drug-induced exanthems?

It is often difficult to distinguish but here are some clues ...
  • viral exanthems often have associated fever, malaise, sore throat and conjunctivitis; although drug reactions may have these features also
  • viral exanthems happen more frequently in children; although adults can get them too 
  • a rash that develops within 3 days of a drug being initiated for an infection is more likely secondary to the infection because of the time required for hypersensitivity to develop in a patient not previously sensitized to a particular drug

Selected infections and Other Conditions that Often Include an Exanthem and Characteristics that Help Differentiate Them from an Exanthematous Drug Eruption.  (click image to enlarge)

Source

 Stern, R.  "Exanthematous Drug Eruptions"  NEJM.  28 June 2012.