Check this out:
"new study finds that older women who use multivitamins may be more likely than non-users to develop breast cancer"
Is there any chance of a sampling bias in this study? Any chance at all that the population of elderly women who take multivitamins may be different in any health parameters from the population that finds no need for such supplements? Furthermore, is there any chance that the study subjects taking the vitamins are more likely to be examined more frequently and therefore more likely to have cancer detected?
This reminds me of the brilliant study (cited from the grocery store tabloids by my sister-in-law) that diet soda makes you fat, because a study found that people who drink diet soda were more likely to be overweight than those who do not drink it.