Are multivitamins such as centrum and one a day bad for you?

i went to gnc this afternoon for more protein shake. the salesman there asked me if i took a multivitamin and i said yes i used to take centrum, now i take teenage one a day for him, and he said they are not good for me. he said tht u would only get like 20% of vitamins tht u need, and tht there are carcinogens in those vitamins. is it tru tht multivitamins such as one a day and centrum bad for u? wht mutivitamins are good for u?