Nicotine use over time increases the speed that codeine is converted into morphine within the brain, by increasing the amount of a specific enzyme, according to new research in rat models. It appears smokers' brains are being primed for a bigger buzz from this common pain killer -- which could put them at a higher risk for addiction, and possibly even overdose. These findings are part a new way of seeing the brain's role when it comes to drugs and toxins.