Nielsen S, Van Hout MC (2017). "Over-the-Counter Codeine-from Therapeutic Use to Dependence, and the Grey Areas in Between". Current Topics in Behavioral Neurosciences. 34: 59–75. doi:10.1007/7854_2015_422. ISBN978-3-319-60014-7. PMID26768736.
Nielsen S, Van Hout MC (2017). "Over-the-Counter Codeine-from Therapeutic Use to Dependence, and the Grey Areas in Between". Current Topics in Behavioral Neurosciences. 34: 59–75. doi:10.1007/7854_2015_422. ISBN978-3-319-60014-7. PMID26768736.