Data shows that when the these medicines are misused, it can lead to overdose, especially in children under 2 years old. U.S. Food and Drug Administration reviewers have said that from 1969 through 2006 the agency received 54 reports of deaths with decongestants and 69 with antihistamines. Most were in children younger than 2. Overdose and drug toxicity were commonly reported in those cases, they said.
FDA reviewers have recommended that over-the-counter cough and cold medicines that contain decongestants and antihistamines should come with new instructions saying they are not recommended for very young children. Link