A new study has found that smokers who take multivitamins offset their healthy behaviour by smoking more cigarettes. This is an example of what psychologists call the licensing effect, which occurs when people make a virtuous choice that permits them to make a poor choice later on, such as when someone 'earns' a weekend binge by avoiding alcohol all week. In this case, smokers take multivitamins, a healthy choice that they believe reduces the risk of cancer and allows them to smoke more. In fact, there is no evidence that multivitamins protect against cancer.

The study, published online today in the journal Addiction, describes two experiments run by the authors. In the first experiment, run as a dummy health-food test, 74 daily smokers were given a placebo, but half were told they had taken a Vitamin C supplement. The smokers then took a one-hour unrelated survey during which they were allowed to smoke. Those who thought they had taken a vitamin pill smoked almost twice as much as those who knew they had taken a placebo (the control group) and reported greater feelings of invulnerability.

The second experiment was an expanded version of the first, with 80 participants taken from a larger community and half told they were taking a multivitamin pill. The one-hour survey also contained questions about attitudes to multivitamins. The smokers who thought they had taken a multivitamin once again smoked more than the control group. But this time, researchers found that among the multivitamin group, smokers with more positive attitudes toward multivitamins experienced a higher boost in perceived invulnerability and smoked even more than their less enthusiastic counterparts. In other words, the amount of extra smoking rose if the smoker expressed a conscious belief that multivitamins increased health.

Health-conscious smokers who take vitamins may thus trigger fundamental but false beliefs that they are invulnerable to the major health hazards associated with smoking, which will lead them to smoke more and increase their overall health risk. Says lead author Wen-Bin Chiou, "Smokers who take dietary supplements can fool themselves into thinking they are protected against cancer and other diseases. Reminding health conscious smokers that multivitamins don't prevent cancer may help them control their smoking or even encourage them to stop."

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