Using pain treatments which contain codeine may be risky for some breastfeeding mothers, according to researchers at The University of Western Ontario, and the Hospital for Sick Children (SickKids) in Toronto. Lead author Dr. Gideon Koren published research in the journal, Clinical Pharmacology & Therapeutics which suggests that the codeine used in some pain relief drugs can actually have harmful and even fatal results for infants when ingested by some breastfeeding mothers.