The brain receives huge amounts of information from outside our body via our five senses (vision, sound, taste, touch, and smell), integrates this information, and orders our muscles to take action. The membrane that makes all this happen so efficiently is called Myelin... *Without this all these things would be much slower and less efficient.*
Try to incorporate all of these at the same time. I think that would be a good mix for all of your students. It also depends on what age your students are. If they are younger they may not be able to understand things that are too hard, so don't be too hard on them either.
Some internal reasons might be to relieve pain, both physical and emotional as Codeine can induce a high. The high is addictive. Another reason would be for the side effects, such as drowsiness which can feel like a high. An external reason would be weight, as the class of drug Codeine is in is an opioid. Opioids can cause weight loss, and unrealistic standards of beauty may give a teen the want to loose weight.