Adverse reaction to pseudoephedrine.

A patient developed a scarlatina-like rash on two separate occasions after receiving a dose of pseudoephedrine. Patch tests with this substance and other structurally related substances (i.e. ephedrine, phenylephrine, and epinephrine) were negative. The oral test with pseudoephedrine provoked a new episode. It is difficult to clarify the exact mechanism of the described reaction; the nature of this eruption probably resembles many other drug-induced adverse reactions in which there is no certainty if mechanisms of type I or III are involved.