Cough and cold remedies sold over the counter often contain combinations of several medications. Research shows that they are not effective when given to children. They can, however, cause serious harmful effects, including accidental overdose, particularly when combined with other medications. For these reasons, since 2008, Health Canada has advised against their use in children less than six years of age.
Sources:
Goldman RD, et al. Treating cough and cold: Guidance for caregivers of children and youth. Paediatr Child Health. 2011 Nov;16(9):564-9. PMID: 23115499.
Mazer-Amirshahi M, et al. The impact of pediatric labeling changes on prescribing patterns of cough and cold medications. J Pediatr. 2014 Nov;165(5):1024-8.e1. PMID: 25195159.
Sharfstein JM, et al. Over the counter but no longer under the radar–pediatric cough and cold medications. N Engl J Med. 2007 Dec 6;357(23):2321-4. PMID: 18057333.
Yang M, et al. Revisiting the safety of over-the-counter cough and cold medications in the pediatric population. Clin Pediatr (Phila). 2014 Apr;53(4):326-30. PMID: 24198312.