Six months later the patient received patch tests with the ingredients of Coldex, ie, 1% and 10% acetaminophen in petrolatum, 1% and 10% phenylephrine in petrolatum, 1.5% phenylephrine in distilled water, 1% chlorpheniramine in both petrolatum and distilled water, and 0.5% caffeine in petrolatum. On day 2 the reading for the patch containing 1% acetaminophen was 3 tiny papules and the other readings were negative. On days 4 and 7 all readings were negative. On day 7, however, the patient developed a symmetric vesiculopustular eruption on the trunk and extremities. Histologic examination of a skin biopsy sample from one of these lesions revealed papillary edema and perivascular infiltrates with neutrophils and eosinophils. The eruption resolved after 1 week. A patch test 1 month later using only 1% and 10% acetaminophen in petrolatum provoked no reaction on days 2, 4, and 7. But on day 6, the patient developed a rash similar to the one that erupted on day 7 of the previous test. That rash had resolved spontaneously by day 10. Separate patch tests with each of the other ingredients of the suspected drug were negative.